File:Eastern Orthodox Church - Kiev Pechersk Lavra (8600767915).jpg

The Eastern Orthodox Church, officially called the Orthodox Catholic Church and commonly 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.

The Orthodox Church 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.

The Orthodox Church traces its development back through the Byzantine or Roman empire, to the earliest church established by St. Paul and the Apostles. It practices what it understands to be the original ancient traditions, believing in growth without change. In non-doctrinal matters the church had occasionally shared from local Greek, Slavic and Middle Eastern traditions, among others, in turn shaping the cultural development of these nations.

The goal of Orthodox Christians from baptism is to continually draw themselves nearer to God throughout their lives. This process is called theosis, or deification, and is a spiritual pilgrimage in which each person strives to both become more holy through the imitation of Christ and cultivation of the inner life through unceasing prayer (most famously, the Jesus Prayer) or hesychasm, until united at death with the fire of God's love.