What if the pope falls into heresy? This was a question that had long perplexed medieval theologians and canonists. The consensus that emerged was that this ecclesiological nightmare could happen. Scholars considered that popes such as Marcellinus and Anastasius II, and even St Peter himself, had fallen into heresy. The difficulty with papal heresy was the idea that the pope had no superior but God. The papal office was divine; hence no individual or institution was considered to have the capacity to judge a pope. According to a traditional argument, a pope who had fallen into heresy ipso facto ceased to be pope and consequently became subject to human judgement. But who should decide that the pope has fallen into heresy, and how?

William of Ockham confronted this problem. In the bull Quia nonnunquam (March 1321), Pope John XXII rejected the Franciscan doctrine of poverty by withdrawing his predecessor Nicholas III's Exiit qui seminat. This attack on the Franciscan doctrine of poverty signified to Ockham that the pope had fallen into heresy. Ockham's anti-papal campaign stands out among contemporary Franciscan responses to the papal sanction. He was the most persistent and thorough in demonstrating that the pope was a heretic. But how was it possible for a theologian and philosopher to justify dissent from a decision made by the successor of St Peter?

The canonists' solutions to the problem of papal heresy are well known.