When the Roman Empire got big it was split into two halves with two emperors. One ruled the west in Rome and the other ruled the East from Constantinople. While the Christian Church was one entity they were starting to split along the same lines with the Pope in Rome and the Patriarch in Constantinople. A real schism occurred in 1054 AD over some doctrinal issues that caused the west to excommunicate the Patriarch in Constantinople who in turn excommunicated the Pope. This led to the current Catholic Church in Rome and Eastern Orthodoxy in Constantinople. They both finally withdrew the mutual excommunications in 1965 even though there is currently no movement towards a reunion of the churches.