The Orthodox Church was first established as separate from the Catholic Church in 1054, after the East-West Schism. Also known as the "Great Schism," this was the moment in history when medieval Christianity finally separated into two branches.
At the time of the schism, the Western Church was led by Pope Leo IX, whereas the head of the Eastern Church was Michael Cerularius, the Patriarch of Constantinople. The schism of 1054 marked the end of several centuries of estrangement and disagreements between the two churches. Political interests were at the core of the disputes. In 1054, the Pope and the Patriarch of Constantinople excommunicated each other, which resulted in a permanent schism.