The Eastern Orthodox Church, officially called the Orthodox Catholic Church, and also referred to as the Orthodox Church and Orthodoxy, is the second largest Christian church in the world, with an estimated 225–300 million adherents, most of whom live in Eastern Europe (especially Southeastern Europe), the Middle East, and Russia. It identifies itself as the present-day continuation of the theology and episcopacy established by Jesus Christ through his Apostles. It teaches that it is the One, Holy, Catholic and Apostolic Church established by Jesus Christ in his Great Commission to the disciples around 2,000 years ago.
The Church has grown from its original territory to be a truly global religion, with individual churches in most of the countries of the world and almost every major city. With the only permanent church in Antarctica, it is the only church that has spread to all seven continents. It is the religious affiliation 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 Bosnia and Herzegovina, Albania, Kazakhstan, Israel, Jordan, Lebanon and Syria.
The Church's structure is composed of several self-governing ecclesial bodies, each geographically (and often nationally) distinct but unified in theology and worship. John Anthony McGuckin identified four 'ancient patriarchates', Constantinople, Alexandria, Antioch and Jerusalem, and eleven autocephalous churches, Cyprus, Sinai, Russia, Greece, Bulgaria, Serbia, Romania, Georgia, Poland, Albania, and Czech and Slovakia, and three autonomous churches, Finland, Japan and China. Each self-governing body (autocephalous jurisdiction), 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. Orthodox bishops trace their lineage back to the apostles through the process of apostolic succession.
The Orthodox Church traces its development back to the earliest church established by St. Paul and the Apostles, through the ancient Roman Empire and its continuation the Byzantine Empire. It regards itself as the historical and organic continuation of the original Church founded by Christ and His apostles. It practices what it understands to be the original faith passed down from the Apostles (that faith "which has been believed everywhere, always, and by all", namely Holy Tradition), believing in growth and development without alteration of the faith. In non-doctrinal, non-liturgical matters the church has always shared in local cultures, adopting or adapting (conventional) traditions from among practices it found to be compatible with the Christian life, and in turn shaping the cultural development of the nations around it, including Greek, Slavic, Romanian, Middle Eastern, North African, British, Saxon, and Celtic peoples. (For an example, see Yule log).
Through baptism, Orthodox Christians enter a new life of salvation through repentance, whose purpose is to share in the life of God through the work of the Holy Spirit. Christian life is a spiritual pilgrimage in which each person, through the imitation of Christ and hesychasm, cultivates the practice of unceasing prayer (often with use of the Jesus Prayer). This life occurs within the life of the church as a member of the Body of Christ. It is through the fire of God's love in the action of the Holy Spirit that the Christian becomes more holy, more wholly unified with Christ, starting in this life and continuing in the next. Born in God's image, each person is called to theosis, fulfillment of the image in likeness to God. God the creator, having divinity by nature, offers each person participation in divinity by cooperatively accepting His gift of grace.
The Orthodox Church, in understanding itself to be the Body of Christ, and similarly in understanding the Christian life to lead to the unification in Christ of all members of his body, views the church as embracing all Christ's members, those now living on earth, and also all those through the ages who have passed on to the heavenly life. The church includes the Christian saints from all times, and also judges, prophets and righteous Jews of the first covenant, Adam and Eve, even the angels and heavenly hosts. In orthodox services, the earthly members together with the heavenly members worship God as one community in Christ, in a union that transcends time and space and joins heaven to earth. This unity of the Church is sometimes called the communion of the saints.