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East–West Schism to Reformation (1054–1517)

With the East–West Schism, the Eastern Orthodox Church and the Catholic Church split definitively in 1054. This fracture was caused more by political events than by slight divergences of creed. Popes had galled the Byzantine emperors by siding with the king of the Franks, crowning a rival Roman emperor, appropriating the Exarchate of Ravenna, and driving into Greek Italy.

In the Middle Ages, popes struggled with monarchs over power.

From 1309 to 1377, the pope resided not in Rome but in Avignon. The Avignon Papacy was notorious for greed and corruption. During this period, the pope was effectively an ally of the Kingdom of France, alienating France's enemies, such as the Kingdom of England.

The pope was understood to have the power to draw on the Treasury of Merit built up by the saints and by Christ, so that he could grant indulgences, reducing one's time in purgatory. The concept that a monetary fine or donation accompanied contrition, confession, and prayer eventually gave way to the common assumption that indulgences depended on a simple monetary contribution. The popes condemned misunderstandings and abuses, but were too pressed for income to exercise effective control over indulgences.

Popes also contended with the cardinals, who sometimes attempted to assert the authority of Catholic Ecumenical Councils over the pope's. Conciliarism holds that the supreme authority of the church lies with a General Council, not with the pope. Its foundations were laid early in the 13th century, and it culminated in the 15th century with Jean Gerson as its leading spokesman. The failure of Conciliarism to gain broad acceptance after the 15th century is taken as a factor in the Protestant Reformation.

Various Antipopes challenged papal authority, especially during the Western Schism (1378–1417). It came to a close when the Council of Constance, at the high-point of Concilliarism, decided among the papal claimants.

The Eastern Church continued to decline with the Eastern Roman (Byzantine) Empire, undercutting Constantinople's claim to equality with Rome. Twice an Eastern Emperor tried to force the Eastern Church to reunify with the West. First in the Second Council of Lyon (1272–1274) and secondly in the Council of Florence (1431–1449). Papal claims of superiority were a sticking point in reunification, which failed in any event. In the 15th century, the Ottoman Empire captured Constantinople and ended the Byzantine Empire.