Several events in history have been referred to as the “Great Schism,” but most people use the term in reference to the East-West Split, a major event in the history of Christianity. It is also common to hear the Western or Papal Schism of the 14th century referred to as the Great Schism. In both cases, these schisms represented a fundamental change in the nature of Christianity, and a marked departure from the traditions and doctrinal beliefs which existed prior to the schism.
The roots of the East-West Split are ancient, almost as old as Christianity itself. Many historians believe that this Great Schism can be dated back to the second century, although matters didn't build to a head until 1054. However, for observant Christians, especially in the higher ranks of the Church, the writing was on the wall, sometimes literally.
The fundamental split between the Roman and Orthodox Christian churches was triggered by several things. Obviously, doctrinal disputes played a huge rule, especially the filioque controversy, a bitter argument which erupted over the addition of “and Son” to the Nicene Creed. The two churches also faced a language gap, with Greek being spoken in much of the Orthodox world, while Latin was spoken in much of Europe. East and West also clashed over papal authority, the use of icons, and a number of religious issues.
In 1054, a deputation from Rome arrived in Constantinople to pressure Michael Cerularius, the Patriarch or religious leader of Constantinople. The goal was to force Cerularius to acknowledge papal authority, in the hopes of reconciling the widening gap between the East and the West. Cerularius, however, refused to accept the authority of Rome, so the legates handed him an order of excommunication written by Pope Leo IX in anticipation of this very situation. Cerularius responded by excommunicating the legates and the Pope, triggering the Great Schism.
Technically, only a handful of people were affected by the excommunications, but they summed up a larger dispute between the Churches, leading Christians to take sides. In the East, Christians sided with Constantinople, viewing it as a center of their faith and rejecting Roman authority, while in the West, Christians cleaved to the Pope, their traditional authority figure. Despite attempts at reconciliation, the Great Schism created a breach which could not be repaired.
Over the centuries, relations between East and West have varied considerably. In the 20th century especially, genuine reconciliation began to be widely reported, with Papal visits to Orthodox nations and other attempts to reach out. However, the fundamental chasm between the faiths created by the Great Schism is unlikely to be repaired, although both sides today may express mutual respect.