The most profound crisis of authority that arose in the late Middle Ages affected the papacy: the Great Schism which lasted from 1378 to 1417 was by far the worst problem which the medieval church had to face. This Schism served as a focus for the development of two sets of ideas concerning power and legitimate authority – both sets had their origins before it. These ideas were different in kind and both were potentially deadly threats to papal claims to plenitude of power: the theory of grace-founded dominium and the theories of the conciliar movement. The origins of both lay before the Schism, but their time in the sun came during it.

The Great Schism posed a worse threat to the papacy than any experienced during those other watershed crises, the Investiture Contest or the conflicts between Philip IV and Boniface VIII. What was at stake was nothing less than an institutional breakdown in the governing structure of the church. Gregory XI had brought the papacy back from Avignon to Rome in January 1377, but died in March 1378. On 8 April 1378 the Archbishop of Bari, Bartolomeo Prignano, was elected pope in Rome and took the name of Urban VI. He was initially accepted as pope by the cardinals, but they became increasingly convinced of his unsuitability, largely because of his behaviour towards them. The result was that the self-same college of cardinals which had elected Urban declared that he had been uncanonically elected and that therefore the papal throne was vacant. On 20 September 1378 at Fondi, the cardinals elected one of their number, Robert of Geneva, as pope, who took the name of Clement VII. Mutual excommunications between Urban and Clement followed. Clement returned to Avignon where the bulk of the papal curia had remained all along. There were thus now two claimants to the papacy, a Roman and an Avignonese one. There had been many schisms before in the medieval church, but what was new about this one was that the same college of cardinals had elected two popes within the space of a few months.