The East-West Schism, also known as the Schism of 1054, was an event that prompted the ultimate split between the Eastern Christian churches (headed by Constantinople's patriarch, Michael Cerularius) and the Western church (led by Pope Leo IX). It resulted from disputes over who had authority to appoint bishops, as well as other issues.
The schism began when Emperor Constantine X Ducas ordered the deposition from his throne of Peter I, the patriarch of Constantinople. The action was taken because Peter had supported another candidate, John Papylus, for the crown against Constantine's preferred candidate, his nephew Alexios II. As a result, on August 15, 1054, Constantine declared that all things done at Rome were invalid and that no Roman bishop could hold any rank within the Eastern Church. This decision was confirmed by another imperial decree two years later.
Meanwhile, in Europe, Pope Leo IX was attempting to resolve the dispute between the Eastern and Western churches by sending envoys to Constantinople to negotiate a reunion. However, these efforts were unsuccessful and so, to avoid further conflict, both sides decided to divide up the Catholic world with neither side having jurisdiction over those regions they had separated from one another. Thus, Russia became part of the Eastern Church while Italy, France, and most of Spain joined the West. Although Greece remained under Ottoman rule, it too was divided into Western and Eastern halves.
The Great Schism divided Christianity's main factions into two camps: Roman Catholic and Eastern Orthodox. They are still the two major Christian denominations today. Michael Cerularius, Patriarch of Constantinople, was excommunicated from the Christian church established in Rome, Italy, on July 16, 1054. This act constituted the beginning of the separation between those churches.
The cause of the schism was a conflict over who should have the authority to appoint new bishops. When Pope Leo IX died in 1049, there were no clear rules about who should follow his lead. The matter was resolved when Cardinal Humbert of Silva Candida (now Italy) was elected by the Romans to be their bishop. But Humbert was sent back to continue his career at the papal court where he became one of the most influential cardinals of his time. He is now known as Hubert Walter.
Meanwhile, in Constantinople (today's Turkey), a new pope had been elected but he did not last long. His name was Clement III and he was a French monk named Pierre. During his short reign, many problems began to arise with the Christians living under Muslim rule. The Muslims found it difficult to accept that a slave-born priest could become head of the Church and so they attacked him every chance they got until they finally killed him in 1054.
Historians consider the reciprocal excommunications of 1054 to be the apex event. It is impossible to agree on a precise date for the incident that marked the beginning of the split. It might have begun as early as the Quartodeciman debate during the reign of Victor of Rome (c. 180). The end of this era would have been before the close of Gregory VII's pontificate in 1085.
The mutual excommunications were a declaration by the churches of East and West that they were no longer in communion with one another. They were issued by Pope Leo IX on 18 September 1049 and by Emperor Conrad II on 2 December of the same year. The exact dates are significant because they mark the beginning of an interim period where no official communication was permitted between the two parts of Christendom. During this time, negotiations were conducted with the aim of restoring peace between the churches, but without success. It was not until 1054 that Pope Reginald II of Rouen and Emperor Henry III of Bavaria agreed on a treaty by which they pledged to restore peace between their churches.
In English law, the term "mutual exclusion" refers to the situation when two parties, by their acts or by their default, exclude themselves from a relationship with each other. In Christian theology, the term refers to the permanent separation of East and West into ecclesiastical groups after both had condemned the other's leader.