(Reproduced with the editor's permission from The Journal of Religious History, 16 (1991), pp. 387-409; revised. (JHI is published by Blackwells)
During the night of May 26th, 1328, Michael of Cesena, Minister General of the Franciscan Order, accompanied by several members of his order including William of Ockham, fled from the court of pope John XXII at Avignon.  For some time John had been in conflict with 'spirituals' or 'zealots' among the Franciscans and at first had had Michael's cooperation; indeed Michael had handed over to the Inquisition certain 'spirituals' who refused to promise obedience to one of John's rulings, and four of them had been burned. But John's opposition to the doctrines of the spirituals caused him eventually to antagonise Michael and other leaders of the order.  In a series of statements published between 1322 and 1324 he rejected teachings on the place of poverty in Christian life that had been formulated by Franciscan writers and endorsed by pope Nicholas III in his decretal Exiit qui seminat (1279). John taught that no one can renounce property rights altogether, as the Franciscans claimed to do, since the use of things consumed in use (such as food and drink) presupposes property in them; he therefore condemned as heresy the Franciscan doctrine that Christ and the apostles had had no property. He also contradicted certain traditional doctrines of property. Theologians commonly held that property came into existence after Adam's sin, by human enactment; according to John, property exists by divine law and not by human enactment, and Adam had property in the garden of Eden.  Believing that in these matters John had become a heretic and had therefore ceased to be pope,  Michael and his followers left Avignon and went into opposition. John excommunicated them, and most of the Franciscan order then submitted to the authority of superiors obedient to the pope. Within a few years John gave the Michaelists further reason for regarding him as a heretic when he preached in a sermon that the saints are not already in heaven, but will enjoy the beatific vision only after the resurrection of the body.  His successor, Benedict XII, condemned this doctrine, but did not reverse John's teaching on poverty and property, and the Michaelists regarded Benedict also as a heretic.
John had also come into conflict with the 'Roman Emperor', Ludwig of Bavaria. In an election held in 1314 the imperial electors had split, a small majority supporting Ludwig and the others Frederick of Austria; there was no accepted rule that a simple majority would suffice, and for several years indecisive fighting took place. Pope John recognised neither of the rivals. Their strife suited him well: while they fought in Germany they could not frustrate his policies in Italy. He repeated the claims made by some of his predecessors that while the Empire was vacant its jurisdiction belonged to the pope, and that without papal approval an Emperor-elect could not become Emperor. In 1322 Ludwig defeated his rival in battle, but John still refused to recognise him, and excommunicated him (1324) for using the imperial title and exercising imperial rights without papal approval. John and his successors were never persuaded to lift the excommunication. Ludwig died excommunicate in 1347. 
After leaving Avignon Michael and his companions made their way to Ludwig in Italy and then returned with him to his court at Munich.  From there during the remaining twenty years of his life Ockham sent out a stream of works intended to convince Europe that John XXII and his successor, Benedict XII, should be deposed because they were heretics and because they had usurped the rights of the Empire. These writings include the Work of Ninety Days, a critical commentary on John's defense of his views on poverty and property against the criticisms of Michael of Cesena, the Dialogue, in which a Master explains to his student the views of various schools of thought on the questions disputed between John XXII and his opponents, and Eight Questions, on various questions of Church and state.  These are all 'recitative' writings, in which the author does not state his own opinion but presents and compares the opinions of others. Ockham also produced a number of 'assertive' writings -- Letter to the Franciscans, Against John, Against Benedict,  Short discourse on the tyranny usurped by some called Supreme Pontiffs,  and On the power of popes and emperors  -- in which he argued directly that John and Benedict were heretics and tyrants who should be deposed. The assertive writings were not widely circulated, but the recitative writings were, and were printed in the late fifteenth century. [ 14] These works helped disseminate ideas on civil and religious polity which were influential during the period of the Great Schism (1378-1418) and the council of Constance (1414-18), and during the political and religious conflicts in sixteenth century France and seventeenth century England. On what Ockham regarded as the main issue, the allegation that John and Benedict were heretics, his campaign was a complete failure. The names of John and Benedict have never been deleted from the list of popes.
Ockham himself was still excommunicate when he died in 1347.  For a long time Catholics had hardly a good word to say for him: he was the arch-nominalist, the sceptic, the destroyer of the scholastic synthesis, the inventor of an individualistic theory of personal rights, an encourager of laicism, a forerunner of Luther. More recent scholarship plays down his influence and gives his his work a less radical interpretation. 
But another accusation has been made. Professor Brian Tierney's book on the history of the doctrine of papal infallibility  includes a chapter, 'Anti-Papal Infallibility: William of Ockham', in which he argues that Ockham was 'an early champion of the doctrine of papal infallibility' (p. 205). 'Infallibility can be a corrosive concept in Catholic ecclesiology if all its implications are explored both rigorously and audaciously. Ockham's work demonstrates this truth in the highest degree' (pp. 209-10). According to Professor Tierney, Ockham's attempt to explore the logical implications of the concept of infallibility leads to 'a tangle of self-contradictions' (p. 227). He 'seems constantly on the brink of reaching sensible conclusions but never quite succeeds in doing so' (p. 234). He falls into 'a morass of total subjectivity' (p. 228). 'In the end Ockham's conclusions were simply perverse... He offers us only dogma without order, anarchy without freedom, subjectivism without tolerance. And in a real sense it was ockham's obsession with the idea of infallibility that drove him to such conclusions' (pp. 235-6).
Ockham was not a champion of papal infallibility. In fact he argues that no part of the Church is infallible. And, in my opinion, his thinking about the Church is coherent, sensible, and not at all perverse. In this paper I will present an interpretation of it which I believe is immune to Professor Tierney's criticisms. 
However, there is a preliminary question of method. Since some of Ockham's most important works are 'recitative', there is some doubt whether they can be used as sources for his own views. According to Professor Tierney,
One can prove anything about Ockham by simply ascribing to him the opinions expressed by the Magister in this treatise [the Dialogus]. We shall therefore follow the rule of never attributing to Ockham the views expressed in the Dialogus unless there is evidence from his other writings that he actually held an opinion presented there (p. 206).
In my opinion this rule is too restrictive. It is true that the Dialogue presents opposed opinions and can therefore be quoted in opposite senses, but it is not impossible, or even very difficult, for readers familiar with Ockham's work and have an understanding of the logic of his situation as a controversialist to tell which are Ockham's opinions and which are not. I suggest that opinions found in the Dialogue, especially those which the Master explains carefully, can safely be attributed to Ockham if they are not refuted or strongly objected to in other passages, if they are consistent with opinions expressed in his 'assertive' works, and if they support the purposes Ockham pursues in those works. Further, since on each of the questions discussed in the Dialogue Ockham's own opinion is one of those presented (Dial., 1r a36-8), where only one is presented (as is the case in much of Dial., 1, iv (22r b46-32v a55), important on the subject of heresy) it must be Ockham's.  I will therefore make use of the Dialogue and the other recitative writings.
One of the foundations of Ockham's thinking about the Church is Christ's promise in Mt. 28:20, 'I will be with you all days, until the end of the age'; see CB, 261.12, Dial., 36v a13-21. He seems to take 'all days' to mean 'each and every day', and applies the promise to 'the Church living at the same time in this mortal life', which he also calls 'the Church militant' (i. e. on active service -- see 2 Tim. 2:4). [ 22] In interpreting the promise Ockham assumes something like his razor: just as in natural philosophy no entity should be postulated beyond what is needed to 'save' the phenomena, so here we must not suppose that the promise commits God to anything more than is needed to fulfil it, though he may in fact do more. The least that must happen to save Christ's promise to be with his disciples all days (at least, insofar as it relates to the faith of the Church) is that in each and every 'day', if anyone tries to inculcate an error as something Catholics must believe, then there must be at least one member of the Church who resists (CI, 67.22-7; EFM, 15.23-7; Dial., 14v b15-53). There is no warrant in this promise, or otherwise, for believing that any particular part of the Church (such as pope, cardinals, general council, or the majority of Catholics) cannot fall into heresy. 'What is promised to the whole and not to any part ought not to be attributed to any part, even to a more principal part' (43v a8-10; cf. lines 45-54; also 34v b25-30, 38r b27-39, 46r a33-42, 48r a29-38, 48v b23-36, 50r b15-28).
The promise does not guarantee positive acceptance of the truth by the whole Church, or by some organ representing the whole Church; what it guarantees is public rejection by at least one member of the Church of any error being put forward in that day as Catholic truth. Ockham does not say that the whole acting as a whole is infallible, but that God will not allow the whole, i.e. every member at once, to acquiesce in error:
In the Church militant there is certain judgment with repect to the things which must be believed explicitly to attain eternal salvation... But with respect to things not necessary to believe explicitly there need not always be certain judgment in the Church militant. For there are many things concerning which God-fearing doubt is better than rash affirmation of one or the other part of the contradiction. But concerning all such Catholic truths it will never happen that all Christians pertinaciously err or doubt; there will always be some in the Church who, concerning such things, at the opportune time and place seek the truth with careful labour, ready also to hold it explicitly if they find it, whether by their own meditations, or by some occasion taken from the scriptures, or from other men (no matter whom), or through divine revelation (Dial., 47r b53-v a15; cf. 209v b43 ff and 211v).
So all Catholics of the day might believe some error on a matter about which explicit faith is not necessary, but at least one will believe it without pertinacity, and will resist (perhaps merely as premature) any attempt to define it as Catholic truth.
It follows from Christ's promise, then, that if some proposition is being taught generally as a truth which all Catholics must believe and none of the Catholics of the day protests that the proposition is an error, or at least that it should not be imposed as something Catholics must believe, then it is indeed Catholic truth. It follows also that if at some 'day' in the past some proposition was taught as Catholic truth without protest, then that proposition is Catholic truth even if some who are or seem to be Catholics now reject it: otherwise the Church of that past day would have erred. 
Some apparently wished to use Augustine's statement that he 'would not believe the gospels except that the authority of the Church compels it' to prove that the pope (representing the Church) has greater authority than the gospels; see CB, 257.24-8, Dial., 1v a36-45. Ockham counters this with another text in which Augustine speaks of the authority of the Church through the ages from the Apostles to our own day: it is this Church, Ockham suggests, which is of more authority than the gospels -- 'not because there is anything doubtful about the Gospel, but because the whole is greater than its part'. From the greater authority of the whole it cannot be inferred that the authority of one part is greater than that of another part -- for example, that the authority of the pope is greater than that of the evangelists; there is nothing doubtful about the gospels, and they are in fact of greater authority than the pope and canon law, as popes themselves have said (Dial., 2v b33-7; cf. CB, 260.21-261.9). In another place Ockham uses Augustine's text to meet the objection that it is not possible for the whole Church to approve doctrine, since it cannot meet: the answer is that the Church through the ages has authority, as Augustine says, but cannot meet; the whole approves when no part objects (CI, 65.30-66.27). In other places the authority of the Church through the ages is used as a reiterated application, so to speak, of the authority of the Church of the day. Any doctrine widely taught and not opposed through the ages since the apostles -- or for any long period -- is guaranteed by the inerrancy of the Church of each 'day' during that time. Thus, if the doctrine of Peter's primacy has been recognised continuously and without dispute since the times of the apostles it must be true (Dial., 227v-8r). If it were accepted unanimously during some time that would be enough to guarantee its truth, but if (as Ockham claims) it was accepted by all Catholics during the whole of this time then its truth is proved abundantly.
The authority of the Church through the ages does not seem to me to have much practical importance in Ockham's thinking; I do not think he would have introduced it except to gloss Augustine's text about the Church compelling belief in the Gospel. At all events, the proposition that the Church through the ages cannot err does not supersede or qualify the proposition that the whole Church in any given day cannot err. After explaining that the Church which compels belief in the Gospel is the Church through the ages, he writes:
But it must be noted that although after the Apostles no Church living at the same time in this mortal life is of greater authority than the Gospel, nevertheless the whole Church, in every one of its persons, cannot err against the faith, because of Christ's promise in the last chapter of Matthew, "I will be with you all days, until the end of the age". What is believed by the whole congregation of the faithful without exception must therefore be held as indubitable, because the whole congregation of the faithful existing at one time in this mortal life cannot err (CB, 261.10-17).
Just as there is no doubt about the Gospels even though the authority of the Church through the ages (including the Apostolic age) is greater, so there is no doubt about any doctrine accepted by every member of the Church alive at one time. 
The 'rule of faith', determining what must be regarded as Catholic truth, consists in 'sacred scripture and the teaching of the whole Church, which cannot err' (CI, 72.35). The Bible is both rule of faith and source of Catholic truth, but not the only source: there are two others, namely apostolic tradition and post-apostolic revelation; see Tierney, pp. 221-2. (Ockham does not know of any examples of post-apostolic revelation, but he says that God can make new revelations if he pleases; Dial., 14v b10-15.) The teaching of the whole Church is not itself a source, but a guarantee that a doctrine comes from one of the sources even if its derivation cannot be shown. If there was ever a 'day' when it was accepted throughout the Church (even if it has been disputed since) then it is certain that it is Catholic truth even if cannot be proved from the Bible, indeed even if it seems to contradict the Bible (CI, 73.20-32).
There is a rule of faith because Catholics are not at liberty to believe, as being truths of faith or 'Catholic truths', any except truths that God has revealed to the Church. Ockham often quotes or alludes to 1 Cor. 2:5: 'Your faith should not stand in the wisdom of men, but in the truth of God' (see CI, 106.28; CB, 218.27, 265.7-8; OND, 853.273; Dial., 16v a39-42, 29r b51-3). This text, with Mt. 28:20, is at the foundation of Ockham's theory of the Church. Undoubting faith is to be given only to God. Popes and general councils do not make new Catholic truths or heresies but only acknowledge what is already truth or heresy (CB, 218.21-32; Dial., 10r a33- v b44, 14v a22-7; OND, 643.25-44). A pope about to define some doctrine must make sure that it has been revealed, and must make sure of this personally:
Although the Supreme Pontiff can delegate many cases, and can even entrust to orthodox persons the examination, discussion and definition of faith, as it seems to many: still, if afterwards he wishes to give solemn and particular approval under his own form to their decision or advice or judgment and insert it in one of his constitutions, he should himself, in person, most diligently examine that decision, advice or judgment, and see every word with full deliberation. And however much he does not see in it anything against the truth, unless in addition he knows clearly that it is founded in Catholic truth, he should by no means insert that decision, advice or judgment in his constitution or solemnly approve it... lest, that is, the determination of a question of faith should rely on the wisdom or will or leading or insistence of men (CI, 107.16-32).
A new doctrine not explicit or implicit in the sources as they stand or in existing Church teaching cannot be accepted as Catholic without a new revelation, and a new revelation may not be accepted unless confirmed by miracle. One possible miracle is divinely-inspired acceptance by absolutely all members of the Church of the day. But the acceptance must be unanimous: 'if even one should dissent such a truth is not to be accepted, since it is possible for the whole faith of the Church to stand in one person alone' (Dial., 14v b41-3) -- that is, the one protestor may be right.
Catholics must have explicit belief in some Catholic truths (CI, 45.35-40, 46.18-22; cf. Dial., 171v a8-13). But it is enough to believe the others implicitly -- that is, to have explicit belief in premisses which imply the rest. 'A Catholic is said to believe implicitly all things contained in divine scripture because he believes explicitly that all things taught in divine scripture are true' (CI, 46.20-22; cf. Dial., 18r a10-12). 'For the sake of those who follow the doctrine of Thomas Aquinas' (who included John XXII) Ockham quotes Summa, 2-2 q. 2 a. 5:
With respect to the primary objects of belief, the articles of faith, a man is obliged to believe explicitly, just as he is also obliged to have faith. With respect to the other objects of belief a man is not obliged to believe explicitly but only implicitly, or in the preparation of the mind, inasmuch as he is ready to believe whatever is contained in divine scripture. He is obliged to believe such a thing explicitly only when it has become known to him that it is contained in the teaching of the faith (Thomas Aquinas, quoted CI, 46-7).
Anyone who happens accidentally to read in the Bible that Bilhah was the mother of Dan is then obliged to believe it explicitly, knowing that it is contained in the Bible; someone else who does not know this will not be obliged to believe it (CI, 50.8-15; cf. Dial., 171v b18-49).
The opposite of a Catholic truth is a heresy, and a heretic is someone who was a Catholic but has ceased to be one by rejecting Catholic truth. But not all who believe heresies are heretics (CB, 264.27-31, 216.9-21; Dial., 4v b3-5, 8r a4-17, 21r b5-20, 28r a12-26; OQ, 214.320-2.) To turn a Catholic into a heretic requires, besides mis-belief, pertinacity (CI, p. 51.17-35).  The opposite of pertinacity is readiness to be corrected by the Catholic faith, i.e. willingness to find out what is Catholic truth and to conform one's thinking to it. Pertinacity may not be a deeply rooted disposition but merely a matter of present intention; someone who at present does not intend to believe whatever is Catholic truth is pertinacious and a heretic, even if he may later change his mind (CB, 321.21-30, Dial., 24r b10-25). A heretic is someone baptized as a Catholic who pertinaciously denies, doubts, or affirms the opposite of, some Catholic truth. (Doubt may make a heretic; see CI, 52.20-30, 58.12-59.34.) Implicit faith is not destroyed by error provided the error is not pertinacious, i.e. provided the one in error seeks Catholic truth and would abandon this error if he knew it conflicted with Catholic truth. The counterpart of implicit faith is 'unknowing' heresy: a person who pertinaciously believes something he does not realize conflicts with Catholic truth is indeed a heretic, though he considers himself a Catholic -- he is pertinacious because if he knew of the conflict he would reject Catholic truth (Dial., 22v b15-29).
Many heretics also have been condemned by general councils who, however, firmly held that what is handed down in sacred scripture is true; but because they pertinaciously adhered to some assertion actually contrary to an assertion of divine scripture (though it did not seem so to them) they were none the less to be judged not Catholics but heretics (Dial., 18r a23-8; cf. 22v b44-9; OND, 857.434-43).
Thus it is possible to be pertinacious without realizing it; see Dial., 22v b15-20.
A judgment by one human being that another is pertinacious in heresy is always fallible, since it requires inference about dispositions (readiness to be corrected or the opposite) from outward words or behaviour (Dial., 22r a46-54, 22r b18-34, 25v a44-51). This explains why Ockham always says 'is to be regarded as a heretic', or something similar, rather than 'is a heretic'. That someone is pertinacious may be revealed by 'examination' or 'legitimate correction' -- i.e. by making the experiment of correcting him, to see whether he is ready to change his beliefs to bring them into agreement with the Catholic faith (CI, 52.2-6). Not to correct an error at the mere behest of a prelate does not prove pertinacity, even in an illiterate layman.
In matters of faith subjects are not obliged to adhere to their prelates with undoubting faith, first because then the subjects' faith would stand upon the wisdom of men, and second because, whether from simplicity or ignorance or from pertinacity, prelates can err against faith (Dial., 27r a45-50; cf. 26v b12-19, 27r a23-32).
It is not proof of pertinacity to defend an error 'a thousand times' (Dial., 29r a18-28), even without explicitly professing readiness to be corrected (Dial., 21r a18-37). Before he can be judged pertinacious the conflict between his belief and the Bible or the teaching of the Church must be shown properly to the person under examination. Ockham supposes that others can sometimes judge that such a conflict has been properly shown.
It is not enough for him to deny that his opinion has been proved heretical, but he is compelled to stand to the judgment of experts, and if they think it has been proved sufficiently that the opinion is heretical, he is obliged to revoke it: otherwise he is to be accounted pertinacious and a heretic (Dial., 29r a28-37; cf. 26v a5-10).
That a person is pertinacious may sometimes be inferred from other indications without the experiment of trying to change his mind. He can be judged to be a heretic if he says explicitly that the Catholic faith is false: 'A person is not ready to be corrected by the Christian faith if he does not regard it as true, certain and sound' (Dial., 20v b54-5; cf. 23v b11-21, 22v b13-5). Also, if he denies some doctrine he can be presumed to know is part of the Catholic faith -- either because everyone knows that it is (CI, 47.22-33; OND, 852.236-46; Dial., 25r b41-v a8), or because of his office, training or previous activity (CI, c. 7, p. 49; CB, 214.11; Dial., 25v b17-32). The presumption may be defeasible: one who has said that the Catholic faith is false may have been in fear of death (Dial., 23v b21-49, 19v b40-20r a15), one who denies what every Catholic is presumed to know may be able to prove that he did not actually know it.
Disciple: What if he really does not know that such an assertion belongs to the Catholic faith? Wouldn't he be excused with God, if he denied it without pertinacity? Master: It is answered that he would be excused with God -- and also with the Church, if he could prove that he did not know that the assertion he denied was published among Catholics as Catholic (Dial., 25r b8-14; cf. 15-21).
Simple people can prove that they did not know by swearing an oath, the educated have to produce evidence from people who know how they were educated (CI, 48.7-30; CB, 214.34-215.9; Dial., 173r b11-2, 19-20). Various other evidences may justify a presumption of pertinacity without examination:
If he protests, swears, asserts or promises, in express words or equivalently, that he will never firmly adhere to the opposite assertion. Again, if he dares to drive others to hold his error or to neglect the opposite assertion by means of sentences, threats, precepts, oaths, or in any way. Again, if he persecutes or molests those who attack the heresy or defend the opposite assertion. Again, if he solemnly defines that his heresy is to be held, or solemnly condemns or argues against the opposite truth. Again, if he refuses to be laudibly informed of the truth. In these ways, some say, and in many others, someone can be convicted of pertinacity; because in these ways and others it is made clearly known that the one speaking the heresy, whether opining or doubting, is not ready to be corrected (CI, 52.6-17).
Thus a pope who solemly defines some error as Catholic truth can be judged pertinacious without examination, since solemn definition implies that his mind is finally made up. To define is 'to assert with constancy and irrevocably', 'determining from final deliberation and compelling others to believe'; if a person purports to define a heresy he need not be examined to see whether he is ready to be corrected -- his obduracy is clear already (OND, 834.105-11, 840.333-4, 850.148-57, 384.164-8).  Ockham has John and Benedict in mind, of course, but he is making a point of general principle applicable to anyone:
He who confirms himself ultimately in some heretical assertion is to be regarded as pertinacious and a heretic, because such a person irrevocably affirms an heretical assertion. But one who defines that it is to be held confirms himself in that assertion through final deliberation (Dial., 32r b41-6; cf. 30r a35-40, 32r a41-5, 32v a16-27).
Anyone, therefore, pope, pretender to the papacy, bishop, doctor, or anyone else, who puts a doctrine forward as something to be held irrevocably is either right or a heretic. On the other hand, if someone puts forward an error in faith or morals without purporting to make an irrevocable definition, he is not to be regarded as a heretic unless he persists after being corrected. Again, this is true of anyone, including the pope.
A pope can err against faith from simplicity or ignorance, but is not to be counted among the heretics; therefore, even if from ignorance or simplicity he deviates from the faith, provided he is ready to be corrected, he is not deprived of the papacy (Dial., 35r a22-6).
A pope, like anyone else, is entitled to propose and defend a view on a contentious question, provided he is ready to be corrected; and, like anyone else, if he asserts it definitively as Catholic truth, he has to be right.
Some of the points above must be glossed, 'at least, in relation to articles of faith that do not have to be believed explicitly'. If a proposition is one which must be believed explicitly doubt or mis-belief is enough, Ockham holds, to make someone a heretic. In such cases doubt or misbelief either (1) necessarily implies pertinacity, or (2) establishes a strong presumption of pertinacity. To elaborate: Perhaps
(1a) pertinacity is not precisely unreadiness to be corrected, but more generally any failure in respect of the duty to believe Catholic truth, including both unreadiness to be corrected and, in relation to articles to be believed explicitly, doubt or misbelief (even with readiness to be corrected). For a definition of pertinacity in such a more general sense see Dial., 22v a26-37. Or perhaps
Whoever speaks... some heresy against a Catholic truth which he is obliged to believe explicitly... is not ready to be corrected. For if he were ready to be corrected he would have done what he is obliged to do to correct himself... But he has not done what he is obliged to do; because he is obliged explicitly to believe the truth opposed to the heresy (CI, 53.6-15; cf. 23-40).
This seems to mean that, in relation to articles to be believed explicitly, the obligation is not merely to make a reasonable effort to find out but even to succeed, and thus to believe what is actually Catholic truth. (Compare Ockham's 'objective' conception of certainty: 'He cannot be certain... unless he explicitly believes the truth' (CI, 104.33-4).) 'Each person is obliged to believe those things explicitly which he ought to be certain are contained in sacred scripture or the teaching of the whole Church' (CI, 49.34-36, emphasis added; cf. Dial., 172r b43-7). Or perhaps
(2) doubt or misbelief on such a point is enough to justify a presumption of pertinacity, in the sense of not being ready to be corrected (in the sense of willingness to make a reasonable effort to find out and to change one's mind).
On interpretation (2) doubt or misbelief in relation to articles to be believed explicitly is simply a case in which examination is unnecessary because of a presumption. Ockham holds that a Catholic is obliged to believe a proposition explicitly
(i) if it is universally published (divulgatur) as Catholic truth,
(ii) if to know that it is Catholic truth is one of the duties of an office he holds in the Church, or
(iii) if he does in fact (perhaps by accidentally finding it in the Bible) know that it is Catholic truth (CB, 214.8-215.8, 216.9-30; CI, chs. 7, 9-12, p. 47 ff; Dial., 171v a51-172r a28).
In cases (i) and (ii) ignorance may be a defence (see above, the passage from Dial., 25r b8-14, and see CI, 60.28-32). This supports interpretation (2) -- on interpretations (1a) and (1b) pertinacity would be inferred in such cases by strict and indefeasible logical implication, and ignorance would be no defence.
But whichever interpretation is right, Ockham holds that anyone who doubts or denies a proposition he is obliged to believe explicitly can be judged to be a heretic immediately, without examination (CB, 215.10-216.4, 217.15-19; Dial., 172r b6-21). In particular, since one who attempts to define has a duty to believe explicitly whatever is Catholic truth on the point to be defined, and to be certain of it, if he defines wrongly he has failed to believe something it was his duty to believe explicitly, and is immediately to be judged pertinacious and a heretic (CI, 104.2-109.21). As Jerome says, better God-fearing doubt than rash definition (Dial., 16r a8-9).
The functions of priests are to say mass, administer the sacraments and teach. Besides their functions as priests, the pope and bishops establish churches, ordain clergy, promote to benefices and provide for the external regulation of Church life, so as to cause worship, the administration of the sacraments and orthodox teaching to take place throughout the world (IPP, p. 466). In exceptional circumstances the pope may also have a part to play in temporal affairs (IPP, p. 466; CB, 296.36-297.14; etc.) By means of ecclesiastical penalties and with the backing of the secular rulers, prelates make sure no heresy is taught (see below). They may temporarily prohibit a teaching without condemning it as heretical (Dial., 14r b25-45, 17r b34-9).
In these ways the work of the pope and bishops is important. But it is not so vital that the Church ceases to exist if they cease to do it. The places of pope and bishops may be vacant for a time or may be usurped by heretics. In the time of pope Liberius, who after assenting to Arian faithlessness held the papacy six years, there was no true judgment of authority and judicial sentence of matters pertaining to the faith (Dial., 47v a16-23). 'A man's body does not remain alive even for a time without its head, but the mystical body of Christ can remain alive for a time without a head' on earth, though it should have an earthly head when it conveniently can (Dial., 45v b31-9). If all the clergy fell into heresy God would make some extraordinary provision for their replacement and would make his will in the matter sufficiently clear if the need arose (Dial., 50r a26-37).
The pope cannot create Catholic truth and has no special role in discovering what it is. His role (a role which general councils also have, especially when the pope is a party to the controversy) is to determine controversies -- that is, to say officially what has been discovered, by ordinary processes of research and reasoning, to be Catholic truth on disputed questions. A pope need not be a theologian; he ought to consult experts, and when they show him that something is in the Bible or has at some time been accepted by the whole Church, then he is bound to believe it (CB, p. 249.21-4); he and the experts would be just as much bound to believe if they had been shown it by a layman (Dial., 29r b45-v a2).
But although papal determinations are arrived at by ordinary theological inquiry, they have more force than the determinations of theologians. The Disciple asks what difference is there between correction by a prelate and correction by anyone else? The Master answers:
In this respect, that the error be laid aside, there is no difference; but in many other respects there is a great difference. For a prelate, and one having jurisdiction over another, can cite him to give an account, and can compel him to hear his instruction, and can compel him to make public revocation, and (if he is found pertinacious and rebellious in respect of the foregoing, and of other things pertaining to the prelate's office) can punish him with suitable penalties. But someone without jurisdiction over one in error cannot exercise these powers over him (Dial., 29v a25-35).
The pope's prerogative above other bishops is that after his decision, definition or assertion no one not certain of the truth can rightly teach the contrary in public even as a mere opinion (Brev., 175.20-3; cf. Dial., 11r a29-32).
When the pope approves something (unless we are certain that he errs against faith or justice) we can and should on occasion approve it, in public and in private and in every way. But that which someone inferior to the pope approves, although we are not certain that he errs, we can, even before others and in public, doubt or deny the opinion and contradict his assertion -- though if he does not err we ought not to assert the contrary pertinaciously (Dial., 251r a11-20).
So once pope or council define something, Catholics are obliged morally and by canon law not to contradict it or bring it into doubt unless they are certain (and they cannot be certain unless they are right) that it is inconsistent with Catholic truth.
Another consequence of papal or conciliar definition is for the functioning of the Church's courts. Heresy is heresy even without formal explicit definition, and all heresies have been formally condemned implicitly or globally (CB, 228.19-21; OND, 295.96-8; Dial, 10v b38-40). But until pope or council condemns a heresy explicitly and in particular, only the pope can procede judicially against those who hold it -- bishops and inquisitors cannot, except by referring the case to the pope; after it is condemned explicitly it can be dealt with directly by the lower courts (CB, 263.31-3; Dial., 29v a35-46; OND, 847.62-93). In these various ways the decisions of popes and councils have consequences not for belief but for outward behaviour, and in this way they regulate theological teaching and discussion among Catholics.
Such decisions also have some consequences for belief, because Catholics should normally presume that papal and conciliar definitions are correct. If a definition is not correct those who know that it is wrong must protest (CI, 67.27-32; Dial., 201v b3-8). But
if something is defined erroneously by a general council, or by a gathering regarded as such by many of the faithful, he who does not know that it errs ought to presume in its favour, but not with so violent a presumption as to exclude the admission of proof of the opposite (Dial., 209r a34-41; cf. 217v a1-5).
Ignorance may excuse the resulting error (CB, 308.32-309.28; OND, 854.314-19). But everyone, even those who accept an erroneous definition, must protect objectors until they have had a chance to prove their objections (OND, 855.362-70; Dial., 65r-69r). Those who do not know that a decision is wrong should presume that it is right, and should not contradict it themselves, but should be willing to listen to those who say it is wrong and should protect them until they have had their say.
It will be seen that Ockham attributes important functions to the clergy. Except for those which they perform by virtue of the sacrament of Orders, they will perform their tasks by ordinary human means, in the same way as leaders of other human organisations do; God may aid them, but he has promised no extraordinary aid. Ordinarily the quality of decisions made by the leaders of the Church will depend on their moral character, intelligence and learning, on the quality of advice available, and on how well they consult and deliberate. Their decisions may be mistaken or unjust, and when wrong they may be opposed and corrected by their subjects, as in Ockham's political theory the wrong acts of all rulers may be opposed and corrected. The Church is of divine institution, but its work (apart from what is done by the sacrament of Order) is done by ordinary natural means.
Let us turn now to Professor Tierney's chapter. He suggests that we can perhaps best pick our way through Ockham's writings on this topic by asking, 'How can a Christian know with certitude the truths of faith that he is bound to know in order to attain salvation?' (p. 210). Ockham's first answer, he suggests, is that authentic papal decrees express Catholic truth with unfailing certainty. But which decrees are authentic? Theologians must examine each decree and determine whether it has been promulgated authentically, by reference to the Bible and the teaching of the Church. But where is there a guide to the true interpretation of Scripture or a sure definition of the doctrine of the Church? Since the whole Church except one member may be in error, a truth of faith can be established with certainty only when no individual member of the Church opposes it; indeed, if the Church through the ages is the authority, to show that a truth is Catholic it would be necessary to show that no single Catholic has ever opposed it. But Ockham does not engage in the historical research that this theory would make necessary: 'He simply projected his own beliefs back into the past and asserted -- with no real pretense at proof -- that they had always been held' (p. 233). 'The true church was by definition constituted by those who adhered to the true faith; and Ockham was convinced of the rectitude of his own position. It followed as a logical implication of his arguments that, if everyone came to disagree with him then he, Ockham, would alone constitute the true church' (p. 236). His search for objective criteria of faith ends in total subjectivity.
It seems to me that Ockham himself did not see any great problem for a Christian in finding out what he must believe explicitly and did not think that papal decrees are the appropriate starting point. On Ockham's account there are some things which a Christian is obliged to believe explicitly precisely because he already knows that they are contained in the Bible or the teaching of the Church. There are other things he ought to know because every Christian is taught them, and then he has been taught them -- or, if not, he is not a heretic if he does not believe them (Dial., 25r b8-14, quoted above). As for things he ought to know because of some office, I suppose Ockham had in mind the things which were ordinarily inculcated as part of clerical education and in the induction to offices in the Church. He did not set unreasonably high standards of knowledge. Doctors of theology and bishops need not remember everything that is in the Bible or in papal or conciliar decrees, they need not have firm belief on doubtful questions of theology (CI, 50.3-12, 57.18-20; Dial., 172r a2-10, 28r a5-10). Jerome was not a heretic although he contradicted the gospel, 'because at the time he did not remember the text of the Gospel of Luke' (Dial., 28r a24-6), though he had translated it. The learning of Christian truth does not begin from papal decrees. A lay Christian learns what is included in the faith, without any anxious search for certainty, in the ordinary course of Christian life -- from parents, by going to church, hearing sermons and so forth. A Christian exercising or preparing for some particular office (doctor, bishop, etc.) may have a duty to study theology -- in part by studying papal and conciliar definitions but primarily by studying the Bible, the Sentences, and the writings of Church fathers and theologians -- and may as a result of study come to know that the Catholic faith includes various propositions, which he will then be obliged to believe explicitly precisely because he does know that they are Catholic truths.
Further, Ockham did not hold the view Professor Tierney ascribes to him, that 'a truth of faith could be established with certainly only when no individual member of the Church dissented from it' (p. 232; emphasis added), or that 'the only truths of religion that we can know with final certitude are those that all Christians have always believed unanimously' (p. 235). It is not his position that certainty requires unanimity in the Church of the day, still less that it requires unanimity in the Church through the ages. There is a logical point here: 'if' is not 'if and only if'. Ockham holds that a proposition is certainly Catholic if (but not only if) in some day, or through the ages, it was taught as Catholic truth and no Catholic opposed it. Those who know of a 'day' when all Catholics agreed about something recently challenged can therefore be sure of it despite the challenge, because the Church of that day could not have acquiesced in an error.  But there are other ways of knowing certainly that a proposition is Catholic truth -- notably, by finding it in the Bible.  'Concerning many questions of faith those learned in sacred letters can be certain of Catholic truth, notwithstanding the question or doubt of anyone else whomsoever' (CB, 250.4-6; cf. 246.3-6, 247.35-9, 253.1-5, 255.34-256.4, 256.24-31). Further, those who doubt or deny Catholic truth do not thereby cease to be members of the Church: they remain Catholics as long as their doubt or denial is not pertinacious. The true Church may therefore contain many people who doubt or deny Catholic truths or hold heresies. So those who read the Bible can be certain of many things, notwithstanding the doubts of denials of others who may be and remain Catholics. And it does not follow that if everyone comes to disagree with someone certain of some point of faith then he must believe that the true Church exists only in him. Ockham certainly says that the Church might some time be reduced to one individual; but that has not happened until all but one not only disagree with the one who holds the truth, but reject the truth pertinaciously. 
Professor Tierney says:
Ockham's failure to formulate any rigid criteria for discriminating between truth and error in documents of the past -- the actual witnesses to the traditional teaching of the church -- left him free to range through the whole field of canonical authorities, selecting and rejecting them at will and imposing his own interpretations on them. Thus Nicholas III's decretal Exiit was immutable and irreformable; Innocent III's decretal Solitae was false and heretical unless a sound meaning could be "wrenched as if by violence" from its words (Breviloquium, p. 45). Both decretals had the same canonical validity so far as any external criteria were concerned (p. 234, note 1).
Ockham certainly does not think that truth is guaranteed by external criteria of the kinds the canonists are usually concerned with -- the legal form of the document, or the procedure by which it was drafted or promulgated (with the advice of the cardinals, or the like), but there are other kinds of external criteria, i.e. standards other than one's own subjective sense of conviction. Whether the doctrine contained in a document is true can be decided (not always with certainty) by consulting external standards such as the Bible, miracles worked in confirmation of new revelation, and any available evidence of the acceptance of the doctrine by Christians (to which papal documents are not the only witnesses). The phrase about 'wrenching as if by violence' is not an admission of arbitrariness. On several occasions when Ockham discusses a papal document he suspects of being heretical he explores the possibility that it may have an orthodox sense (see OQ, 47.71-82, 80.9-12, Brev., 194-7.) In the case of Solitae he does not think that likely -- a non-heretical sense can be got only by forcing the text. He means that Innocent III probably was a heretic.
The title of Professor Tierney's chapter ('Anti-papal Infallibility...'), and his claim that Ockham was 'an early champion of the doctrine of papal infallibility' (p. 205), are misleading. Preservation from error and heresy
should not be affirmed of any person (or body) but one of whom (or which) God has revealed that he (or it) will never err against the faith. Now God has revealed this of the congregation of the faithful, and not of the pope (Dial., 34v b25-30).
The authors Professor Tierney refers to on p. 209 who said that Ockham rejected papal infallibility were right. So is Professor Ryan when he points out that the irreformability of a teaching does not imply the infallibility of the teacher:
Pronouncements and the documents containing them may very well be without error and thus irreformable, but their warrant in that case is the immutability of the faith which happens to be accurately reflected, not an infallibility on the part of the pronouncing pontiff (Ryan, 'Dilemma', p. 38).
As Ryan points out, in almost every place where Ockham refers to the irreformability of a definition he specifies that it has been made 'rightly' (ibid., p. 40, n. 10); he does not assume that a pope is incapable of defining wrongly.
But let us consider an argument in which Ockham may seem to commit himself to a doctrine of papal infallibility. John XXII had claimed the power to revoke certain enactments of his predecessors, and some of his acts Ockham interpreted as inconsistent with doctrines on points of faith and morals found in Nicholas III's Exiit. Ockham argued that this was enough to make John a heretic. He wrote: 'What the Roman pontiffs have once defined in faith and morals... stands so immutably that a successor cannot call it into question or affirm the contrary'. Professor Tierney comments: 'This proposition... provides us with an explicit definition of an extreme doctrine of papal infallibility' (p. 213). Earlier he had said: 'Ockham did insist on the infallibility of the church and on the irreformability of decrees promulgated by the pope as head of the church on matters of faith and morals; and these were the essential elements of the doctrine of papal infallibility that came to be defined in 1870' (p. 209). John's own, heretical, decrees were not promulgated by a pope, since in attempting to promulgate heresy he ceased to be pope, since he thereupon became a heretic (p. 215). But why not suppose that John was orthodox and Nicholas the heretic? Because the doctrine of Exiit was (at least for a time) accepted by the whole Church (OND, 383.120-137, 506.266-280). Ockham's argument seems, then, to be that
(iii) not Nicholas, since his definitions were accepted by the whole Church, which is infallible. So it seems
(iv) that Nicholas, as pope, was exercising the Church's prerogative of infallibility, and for this reason promulgating an irreformable definition -- which amounts to the doctrine of papal infallibility defined in 1870.
However, there is no justification in anything Ockham says for step (iv). The references to the papal office are inessential to his argument. The proposition that a pope who tries to define error ceases to be pope is not designed to guarantee papal infallibility. Ockham never suggests that because a pope cannot succeed in defining an error popes are infallible. It is true that popes cannot define errors -- indeed no one can -- but this is not any sort of infallibility. Proposition (i) is an implication of a more general proposition, that any doctrine anyone attempts to define as Catholic truth must be true, or that person ceases to be a Catholic and thereby loses any office he may have had in the Church (CB, 304.17-8, 30-1, 40, 305.36-7). This proves the infallibility of the pope only if it proves the infallibility of every Catholic, since any Catholic who tries to define error as Catholic truth thereby ceases to be a Catholic (which is why a pope in such a case ceases to be pope -- only a member of the Church can be pope). But of course it does not prove that no Catholic can err, any more than the fact that a bachelor ceases to be a bachelor as soon as he marries proves that no bachelor can marry. Proposition (ii) is again an instance of a more general proposition, that any doctrine accepted by the whole Church must be Catholic truth, no matter who teaches it (per quoscumque vel quemcumque, CI, 67.26). This will not prove infallibility of the pope more than of any author of a statement accepted as Catholic truth by the whole Church. The author of such a statement is not infallible -- his very next doctrinal statement may be false. The only subject to which infallibility can be attributed is the whole Church, because it can accept only true statements. The infallibility of the whole Church can not be exercised through some representative, or even by the Church acting as a whole. The whole Church is infallible only in this sense, that it can never happen that the whole Church, i.e. each and every member living at one time, will accept an error as Catholic truth.
Ockham does not argue that the Franciscan doctrine of poverty must be true just because it is in a properly-made papal decree. When he discusses disagreement between popes he does not suggest that the one definition must be right because it was earlier or because it was made according to the right procedure. Catholics must normally presume that papal definitions are correct. When they conflict, or there is some other reason for suspecting error, what is true has to be decided by examining the Bible and evidence of the universal belief of Catholics. In reference to the beatific vision controversy Ockham writes:
John XXII was bound to believe explicitly what Innocent III asserted of the vision and blessedness of holy souls on account of the authority of divine scripture and the assertion of the whole Church, on which Innocent III's assertion was based... For this is a general rule, that if ever different Supreme Pontiffs are shown to have contrary assertions or opinions about something pertaining to the orthodox faith, to know which is to be preferred to the other it is necessary to have recourse to sacred scripture and the teaching or assertion of the whole Church... For no Supreme Pontiff is the rule of Christian faith, since he can err and fall into heretical evil, dist. 40, c. Si papa, and dist. 19, c. Anastasius. But sacred scripture and the teaching of the whole Church, which cannot err, is the rule of our faith (CI, 72.20-35).
Similarly, John XXII was bound to believe what Nicholas III taught in Exiit because it was well-grounded in scripture (see OND, 845.550 ff) and expressed Catholic truth, as its acceptance by the whole Church proved.
According to at least some versions of the modern doctrine of papal infallibility, there are certain occasions on which God will not allow the pope to teach an error. In the process of theological research and consultation leading up to that occasion God will not necessarily intervene in any special way; errors may be averted at that stage by ordinary human effort. But if, after all, some error survives to the point of final definition, God will in some way, if necessary by a miracle, prevent that definition from taking place. Similarly, Ockham would say that the processes of research and discussion in the Church may eliminate many errors without special divine intervention; but if an error spreads God will in some way prevent its final triumph -- he will not necessarily prevent an erroneous statement on the part of the pope or of a Church council (that is where he differs from modern Catholic doctrine), but he will make sure that not every Catholic acquiesces in the teaching of this error as Catholic truth. In the Dialogue the Master suggests an analogy which well illustrates Ockham's theory as I understand it. If a temporal lord decided to preserve a monastery by making sure it never happened that all the monks were killed, that would not commit him to defending any of them while there were many, but
if all were killed but one, he would defend that one until some other joined him in that monastery, and from then on would leave him to himself; such a lord would preserve the monastery, and yet would preserve no member of the monastery except for a time (Dial., 37r a31-9).
God does not preserve any particular Catholic from error except perhaps for a time, but he preserves the Church from error by making sure that its members do not all fall into error at once.
According to Professor Tierney,
Ockham wanted to see all his enemies punished as heretics. For him the main point of having the true faith defined authentically by a true pope was to facilitate the crushing of dissent by the Inquisition. There was no grain of tolerance in him; he was filled with odium theologicum; he raged incessantly against his enemies. The abusive words "heresy", "heretics", "heretical depravity" are scattered over almost every page of his polemical treatises. As Tabacco rightly observed, if by some twist of fate Ockham himself had ever become pope his enemies would have trembled before the severity of his judgments (p. 235).
Ockham's explicit statement of the treatment he thought due to a pope who has become a heretic is in book vii of Contra Benedictum. After various other matters he comes to the part to be played by the emperor and other lay princes. If a pope is accused of heresy the accusation may be malicious and untrue. The prince should get experts in theology to explain the points at issue and should then form his own opinion. He should not act unless and until he can see for himself that the pope has become a heretic. Then he should take the heretic into custody but inflict no punishment, in particular not put him to death. He should hand him over to a Catholic bishop, or preferably to a true pope, who should degrade him from his clerical status. There is no mention of killing him then.  After degradation he should either be committed to perpetual imprisonment 'or, if he seems truly penitent, be released from custody' (CB, p. 317.27). This does not seem unduly severe, given the medieval belief that a heretic should be punished.
As for the words 'heretic' and 'heretical pravity', it is not surprizing that they appear on every page, considering the nature of the dispute. 'Heretic' is abusive in the way 'communist', 'liberal' or 'conservative' are in various circles abusive, when they are used not only to describe but also to express disapproval. 'Heretical pravity' (wickedness) makes the disapproval explicit; the term was not Ockham's invention, it was commonplace. Ockham certainly regarded John XXII and Benedict XII with disapproval, as men who had betrayed a commitment to hold and defend the true faith, intruders who pretended to supreme authority in a Church whose faith they did not hold. We may not share Ockham's view of the facts. We may dispute the moral principles underlying the evaluation, which make irrelevant the possibility that these popes may have held their beliefs sincerely; but every Christian in the middle ages and early modern times would have endorsed such principles, and some still do.
Ockham never says, as far as I know, that the main point of having the true faith defined was to facilitate the crushing of dissent by the Inquisition. The closest he comes to this, I believe, is when the Master answers the objection that on the theory he has been presenting papal definition will have no more effect than the definitions of doctors: he answers that papal definition has certain legal effects, one of which (and only one) is that unless a heresy has been explicitly condemned by pope or council bishops or inquisitors cannot procede against it (see above). On this view the bishop of Paris in condemning Thomas Aquinas, and the Franciscan order in condemning Peter John Olivi, overstepped the limits of their authority (Dial., 13r a). It seems to be at least part of his concern to keep inquisitors and bishops within the limits of their authority, as defined by theology. It is for theologians to say whether the procedures of heresy trials are in accordance with natural and divine law (Dial., 5v b25-30). References to the Inquisition in the Dialogus are highly critical. The Master reports that some say that inquisitors and prelates proceeding against heresy often act iniquitously and unjustly: illiterate and ignorant, blinded by greed and avarice, pernicious to God's Church, they try to condemn those accused of heresy to acquire their goods; no argument about what ought to be done can be based upon their practice (Dial., 28r a50-5, 29r b33-6). Many points he makes about how it is to be decided that someone is a heretic may imply criticism of unjust practices. It is one of Ockham's purposes to make room for free speech and theological exploration within the Church. 
But only within the Church. Ockham was not a pioneer of general religious toleration. He would never have thought that deliberate rejection of the Catholic faith by someone baptised as a Catholic might be right or excusable, and he never expresses disapproval of the punishment of those who really are pertinacious in heresy. He does not claim freedom of speech for heretics, atheists, Jews or Saracens, but only for those who are (or may possibly be) defending the Catholic faith, until their claims have been tested. He supports the institutions of doctrinal definition and repression of heresy, believing that in many matters relating to the faith there are discoverable standards of orthodoxy to which Catholics have an duty, enforcable even in this life, to conform, and that the prevalence of orthodox belief is important to Church life. This would be enough to motivate a good deal of repression, but nothing beyond what was normally supported by Christians until recent times. There is no evidence, as far as I can see, that Ockham's treatment of those he regarded as heretics would have been especially severe.
Those who, like myself, do not share Ockham's Christian faith can entertain his theory only hypothetically. But to Christians also it must seem open to serious objection. It puts a lot of weight on a doubtful interpretation of Christ's promise 'I will be with you all days'. Is 'I will be with you' a guarantee of the truth of doctrine? Does 'all days' really mean in each and every day? If Christ's presence would allow all but one to fall into error for a time, why might it not allow absolutely all to fall into some less serious error for a time? On the other hand, might it not be expected to do more than preserve only one? And if the promise is about doctrine at all, could it not rather guarantee that copies of the Bible and written records of apostolic tradition and the authentic teaching of the Church will always survive?
But of course a position does not have to be true and free from objection to be reasonable. It seems to me that despite objections that can be made to it Ockham's position is coherent and sensible and worthy of respect. On Ockham's account the Christian's mind is in captivity to Christ, but not to anyone else. All Christians -- prelates and subjects, men and women, clerics and the laity, the learned and the simple -- are entitled to consider any religious question, reach a conclusion and put it forward and defend it freely, even if it is wrong, against pope, bishops, theologians, general councils, or the majority of Christians -- provided they give a reasonable hearing to those who disagree, are ready to correct mistakes, and do not insist (unless they are indeed right) that what they believe is to be held irrevocably as Catholic truth. If the point is important, pope or council can determine what doctrine is to be held irrevocably. Papal and conciliar definitions should generally be presumed to be correct, and will have certain legal and other practical consequences; but if a purported definition is wrong, anyone who knows that it is wrong may, and must, speak out against it. Those who mistakenly attack correct definitions are not necessarily heretics. They can rightly be silenced by those who know they are mistaken, but they are to be corrected and not punished.
However, theological conflict will not be the usual experience of Christians. The main tenets of the faith are clear and settled and known to most Christians, who can readily learn what they must believe from their parents, by going to church, by reading the Bible and by ordinary theological study. They will not generally need to investigate difficult questions. Theologians will discuss finer points, but the Church will not try to settle every question: 'Better God-fearing doubt than rash definition'. Reflection, study and discussion, and even (possibly) new revelations, may lead to the recognition of truths not previously known explicitly, and if there is general agreement without dissent there will be no error. But disagreement and error are possible even among true Christians.
If such ideas had prevailed in the later middle ages the Christian world would have been spared much bloodshed and misery. 
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