The East–West Schism, sometimes known as the Great Schism, divided medieval Christianity into Eastern (Orthodox) and Western (Roman) branches, which later became known as the Eastern Orthodox Church and the Roman Catholic Church, respectively. This was in the period (1378 - 1417) during which the Western church had first two, and later three, lines of popes.

Since the beginning of the 14th century, the papacy had been centered at Avignon. Since France was the leading Catholic nation in medieval times, it seemed natural for a series of French-born popes to administrate Christendom from the city of Avignon and not from Rome. In 1377, however, Pope Gregory XI moved the pontificate back to Vatican City, where he died two years later.

The immediate cause of the schism was the election of Pope Urban VI. After the death of Gregory XI, Roman mobs-fearing the re-removal of the church to Avignon-gathered outside the Vatican to demand the election of a Roman pope. The intimidated cardinals complied by electing the Archbishop of Bari, Bartolomeo Prignani, who styled himself Pope Urban VI. Unfortunately, Prignani was totally unsuited to the position, with a rabid temper that bordered on mental illness.

The cardinals escaped west and elected a new pope. Regretting their choice of Prignani and unable to dislodge him from the papal throne, the same group of cardinals soon elected Robert of Geneva, who assumed the title of Pope Clement VII and reigned from Avignon. The problem was that the Catholic Church now had two popes-each elected by the same (completely legitimate) group of cardinals.

European leaders were forced to take sides. In medieval times, the religious allegiances of citizens were largely determined by the preferences of their rulers. In this case, kings and princes split down the middle: Predictably, France allied itself with Clement VII, along with Scotland, Naples, and Aragon and Castile, while Urban VI was recognized by (among others) England, the Holy Roman Empire, and, of course, northern Italy.

The rift continued past the deaths of both popes. In 1404, the successor to Urban VI, Boniface IX, died in Rome. The Roman cardinals offered Avignon a deal: They would refrain from naming a new pope if the French pope, Benedict XIII, Clement VII's successor, resigned. Avignon declined, and the schism continued as before. Making matters worse, a church council in Pisa elected yet a third pope, Alexander V, who quickly died and was succeeded by John XXIII.

The matter was finally sorted out in 1417. Convening in 1414, the Council of Constance unceremoniously deposed both John XXIII and Benedict XIII (the new Roman pope, Gregory XII, had graciously agreed to resign pending the council's decision). United at last, after prolonged deliberations, the assembled cardinals agreed on the election of Martin V as the new pope of the unified Catholic Church.