On July 16, 1054, Patriarch of Constantinople Michael Cerularius was excommunicated from the Christian church based in Rome, Italy. Cerularius’s excommunication was a breaking point in long-rising tensions between the Roman church based in Rome and the Byzantine church based in Constantinople (now called Istanbul). The resulting split divided the European Christian church into two major branches: the Western Roman Catholic Church and the Eastern Orthodox Church. This split is known as the Great Schism, or sometimes the “East-West Schism” or the “Schism of 1054.”

The Great Schism came about due to a complex mix of religious disagreements and political conflicts. One of the many religious disagreements between the western (Roman) and eastern (Byzantine) branches of the church had to do with whether or not it was acceptable to use unleavened bread for the sacrament of communion. (The west supported the practice, while the east did not.) Other objects of religious dispute include the exact wording of the Nicene Creed and the Western belief that clerics should remain celibate.

These religious disagreements were made worse by a variety of political conflicts, particularly regarding the power of Rome. Rome believed that the pope—the religious leader of the western church—should have authority over the patriarch—the religious authority of the eastern church. Constantinople disagreed. Each church recognized their own leaders, and when the western church eventually excommunicated Michael Cerularius and the entire eastern church. The eastern church retaliated by excommunicating the Roman pope Leo IX and the Roman church with him.

While the two churches have never reunited, over a thousand years after their split, the western and eastern branches of Christianity came to more peaceable terms. In 1965, Pope Paul VI and Patriarch Athenagoras I lifted the longstanding mutual excommunication decrees made by their respective churches.

Today, the two branches of Christianity remain distinct expressions of a similar faith. Roman Catholicism is the single largest Christian denomination, with more than a billion followers around the world. Eastern Orthodoxy is the second-largest Christian denomination, with more than 260 million followers. Eastern Orthodoxy includes national churches, such as the Greek Orthodox Church and Russian Orthodox Church.