The Eastern Orthodox Church, officially the Orthodox Catholic Church, also referred to as the Orthodox Church, Eastern Orthodoxy, and Orthodoxy, is the second largest Christian Church in the world, with an estimated 225–300 million adherents.
The Eastern Orthodox Church is one of the oldest religious institutions in the world, teaching that it is the One, Holy, Catholic and Apostolic Church established by Jesus Christ in his Great Commission to the apostles, and practicing what it understands to be the original faith passed down from the Apostles.
United in communion with the Latin Church before the East–West Schism in 1054, and with the Oriental churches for the first quarter of its history, Eastern Orthodoxy spread throughout the Roman and later Byzantine Empires and beyond, playing a prominent role in European, Near Eastern, Slavic, and some African cultures. Its most prominent episcopal see is Constantinople.
Eastern Orthodoxy has no Papacy or similar authority, but instead teaches that all bishops are equal by virtue of their ordination, and each autocephalous church is typically governed by a Holy Synod. This is one of the main reasons for the division between the Roman Catholic Church and the Eastern Orthodox Church. The commonly-used but unofficial designation "Eastern" derives from the geographical location of the main centers of Orthodoxy in relation to the "Western" churches (now known as the Roman Catholic Church), and from Constantinople being the capital of the Eastern Roman Empire.
Geographically, the majority of Eastern Orthodox Christians in the 21st century reside in Greece, Eastern Europe and Russia, with less numerous communities in the former Byzantine regions of the Middle East and around the Eastern Mediterranean. There are also many small but growing communities in other parts of the world, formed in part through immigration and in part through conversion and missionary activity.