East–West Schism

The East–West Schism, sometimes known as the Great Schism, is the medieval division of Chalcedonian Christianity into Eastern (Greek) and Western (Latin) branches, which later became known as the Eastern Orthodox Church and the Catholic Church, respectively. Relations between East and West had long been embittered by political and ecclesiastical differences and theological disputes. Prominent among these were the issues of "filioque", whether leavened or unleavened bread should be used in the Eucharist, the Pope's claim to universal jurisdiction, and the place of Constantinople in relation to the Pentarchy.

Pope Leo IX and Patriarch of Constantinople Michael Cerularius heightened the conflict by suppressing Greek and Latin usages in their respective domains, involving in the case of Constantinople the closing of all Latin churches in the city and the trampling of the Eucharist consecrated with unleavened bread. In 1054, Roman legates traveled to Constantinople for purposes that included refusing to Cerularius the title of "Ecumenical Patriarch" and insisting that he recognize Rome's claim to be the head and mother of the churches. On the refusal of Cerularius to accept the demand, the leader of the legation, Cardinal Humbert, excommunicated him, and in return Cerularius excommunicated Cardinal Humbert and the other legates. This was only the first act in a centuries-long process that eventually became a complete schism.

The validity of the Western legates' act is doubtful, since Pope Leo had died, while Cerularius's excommunication applied only to the legates personally. Still, the Church split along doctrinal, theological, linguistic, political, and geographical lines, and the fundamental breach has never been healed, with each side sometimes accusing the other of having fallen into heresy and of having initiated the division. The Crusades, the Massacre of the Latins in 1182, the West's retaliation in the Sacking of Thessalonica in 1185, the capture and sack of Constantinople in 1204, and the imposition of the Latin Patriarch of Jerusalem and the Latin Patriarchs made reconciliation more difficult. This included the taking of many precious religious artifacts and the destruction of the Library of Constantinople. According to Catholic historians, what gave concrete reality and permanence to the schism was the setting up of Latin hierarchies in the Crusader states controlled by Westerners, especially Normans, more particularly for the patriarchal sees of Antioch, Constantinople and Jerusalem.

Efforts were made to reunite the two churches in 1274 (by the Second Council of Lyon) and in 1439 (by the Council of Florence). Despite acceptance by the participating eastern delegations, no effective reconciliation was realized, since the Orthodox believe that the acts of councils must be ratified by the wider Church and the acts of these councils never attained widespread acceptance among Orthodox churches. In 1484, 31 years after the Fall of Constantinople to the Ottoman Turks, a Synod of Constantinople repudiated the Union of Florence, making official the position that had already been taken by Orthodox in general.

In 1965, the Pope Paul VI and the Ecumenical Patriarch of Constantinople Athenagoras I nullified the anathemas of 1054, although this nullification of measures taken against a few individuals was essentially a goodwill gesture and did not constitute any sort of reunion between churches. Contacts between the two sides continue: Every year a delegation from each joins in the other's celebration of its patronal feast, Saints Peter and Paul (29 June) for Rome and Saint Andrew (30 November) for Constantinople, and there have been a number of visits by the head of each to the other. The efforts of the Ecumenical Patriarchs towards reconciliation with the Catholic Church have often been the target of sharp criticism from some fellow Orthodox.