The Orthodox Church, also referred to as Orthodoxy, is the second largest Christian church in the world, with an estimated 225–300 million adherents, most of whom live in Eastern Europe, the Balkans, the Eastern Mediterranean region and the Caucasus. It is one of the oldest religious institutions in the world, identifying itself as the original Church founded by Jesus Christ and his apostles, and has played a prominent role in the formation of European and Near Eastern culture. The Orthodox Church traces its history to the church established by St. Paul and the Apostles, through the state church of the Roman and later Eastern Roman (Byzantine) Empires. It teaches that it is the continuation of the One, Holy, Universal and Apostolic Church established by Jesus Christ in his Great Commission to the apostles, encompassing the fullness of the Christian faith. 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).
From its roots in the earliest Christian churches founded throughout the Eastern Mediterranean region, in Palestine, Syria, Asia Minor and Greece, the Eastern Orthodox Church has grown into a global religion, with churches in most of the countries of the world and almost every major city. Its adherents are a majority in Belarus, Bulgaria, Cyprus, Georgia, Greece, Macedonia, Moldova, Montenegro, Romania, Russia, Serbia and Ukraine; significant minority populations exist in other Balkan states (Albania, Bosnia and Herzegovina), in Kazakhstan, the Levant (Israel, Jordan, Lebanon, Palestine, Syria), Iraq, and in Istanbul, the historic seat of the Ecumenical Patriarch of Constantinople. Church liturgies are conducted in a variety of different languages, including the original Koine Greek of the New Testament (in the Greek Orthodox Church), Church Slavonic, and Arabic.
Church administration is composed of self-governing ecclesial bodies, each geographically distinct but unified in theology and worship, including four ancient patriarchates of Constantinople, Alexandria, Antioch and Jerusalem, twelve autocephalous churches, Cyprus, Sinai, Russia, Greece, Bulgaria, Serbia, Romania, Georgia, Poland, Albania and Czech Republic, Slovakia and the United States, and three autonomous churches, Finland, Japan and China. Each self-governing body has a Holy Synod to administer its jurisdiction and to lead the church in the preservation and teaching of the apostolic and patristic traditions and church practices. Orthodox bishops trace their lineage back to the apostles through apostolic succession. Although composed of self-governed bodies, the Eastern Orthodox communion widely regards the Ecumenical Patriarch of Constantinople as the spiritual leader (primus inter pares; first among equals) of the 300 million Orthodox Christians worldwide.
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.