William of Ockham, on Heretics, Books 1-5 and Against John, Chapters 5-16

Theologians and church lawyers in William Ockham's time generally agreed that a pope could become a heretic. According to Ockham, that had happened with Pope John XXII. The first part of Ockham's Dialogue is intended to show that John was a heretic, and to set out what should be done to remove him from the papacy. The relevant questions are discussed in a long conversation between Master and Student in which Ockham's own opinions are not directly stated. In Against John, Ockham makes his views very clear.

According to Ockham, no individual or body within the Church is infallible, not even the pope or a general council. Religious error can spread almost throughout the Church. But there will always be a remnant who do not fall into the error. Thus, a dissident individual or minority may be in the right. Among Christians there should therefore be freedom of speech. Any Christian, man or woman, learned or illiterate, can put forward an opinion and argue for it 'a thousand times', in the face of contradiction by the pope himself, without being a heretic, even if the opinion is in truth a heresy. What makes a believer in a heresy a heretic is pertinacity, i.e. unwillingness to listen or unwillingness to change one's mind even if contrary evidence is clearly explained. A clear sign of pertinacity is an attempt to impose error coercively. According to Ockham, Pope John XXII was a heretic, and therefore no longer pope, because he tried to impose heresies coercively.