The Eastern Orthodox Church, officially called the Orthodox Catholic Church, and also referred to as the Orthodox Church, is the second largest Christian Church in the world, with an estimated 225–300 million adherents, primarily in Eastern and Southeastern Europe and the Middle East. It is the religious denomination of the majority of the populations of Belarus, Bulgaria, Cyprus, Georgia, Greece, Macedonia, Moldova, Montenegro, Romania, Russia, Serbia, and Ukraine; significant minority populations exist in Jordan, Israel, Lebanon and Syria. It teaches that it is the One, Holy, Catholic and Apostolic Church established by Jesus Christ and his Apostles almost 2,000 years ago.

It is composed of several self-governing ecclesial bodies, each geographically and nationally distinct but theologically unified. Each self-governing (or autocephalous) body, often but not always encompassing a nation, is shepherded by a Holy Synod whose duty, among other things, is to preserve and teach the apostolic and patristic traditions and related church practices. Like the Roman Catholic Church, Anglican Communion, Assyrian Church of the East, Oriental Orthodoxy and some other churches, Orthodox bishops trace their lineage back to the apostles through the process of apostolic succession.