The Orthodox Church was founded by our Lord Jesus Christ and is the living manifestation of His presence in the history of the mankind. The most conspicuous characteristics of Orthodoxy are its rich liturgical life and its faithfulness to the apostolic tradition. It is believed by Orthodox Christians that their Church has preserved the tradition and continuity of the ancient Church in its fullness compared to other Christian denominations which have departed from the common tradition of the Church of the first 10 centuries. Today the Orthodox Church numbers approximately 300 million Christians who follow the faith and practices that were defined by the first seven ecumenical councils. The word orthodox ("right belief and right glory") has traditionally been used, in the Greek-speaking Christian world, to designate communities, or individuals, who preserved the true faith (as defined by those councils), as opposed to those who were declared heretical. The official designation of the church in its liturgical and canonical texts is "the Orthodox Catholic Church" (gr. catholicos = universal).
The Orthodox Church is a family of "autocephalous" (self governing) churches, with the Ecumenical Patriarch of Constantinople holding titular or honorary primacy as primus inter pares (the first among equals). The Orthodox Church is not a centralized organization headed by a pontiff. The unity of the Church is rather manifested in common faith and communion in the sacraments and no one but Christ himself is the real head of the Church. The number of autocephalous churches has varied in history. Today there are many: the Church of Constantinople (Istanbul), the Church of Alexandria (Egypt), the Church of Antioch (with headquarters in Damascus, Syria), and the Churches of Jerusalem, Russia, Serbia, Romania, Bulgaria, Georgia, Cyprus, Greece, Poland, Czechlands and Slovak, Albania and America
In the wider theological sense "Orthodoxy is not merely a type of purely earthly organization which is headed by patriarchs, bishops and priests who hold the ministry in the Church which officially is called "Orthodox." Orthodoxy is the mystical "Body of Christ," the Head of which is Christ Himself (see Eph. 1:22-23 and Col. 1:18, 24 et seq.), and its composition includes not only priests but all who truly believe in Christ, who have entered into the Church He founded, those living upon the earth and those who have died in the Faith and in piety."
At the time of the Schism of 1054 between Rome and Constantinople, the membership of the Eastern Orthodox Church was spread throughout the Middle East, the Balkans, and Russia, with its center in Constantinople, the capital of the Byzantine Empire, which was also called New Rome. The vicissitudes of history have greatly modified the internal structures of the Orthodox Church, but, even today, the bulk of its members live in the same geographic areas. Missionary expansion toward Asia and emigration toward the West, however, have helped to spread the presence of Orthodoxy worldwide. Today, the Orthodox Church is present almost everywhere in the world and is bearing witness of true, apostolic and patristic tradition to all peoples.
Excerpt taken and adapted by the Orthodox Christian Information Center from The Orthodox Church by Bishop Kallistos (Ware).
What is the Orthodox Church?
The Orthodox Church is the original Christian Church founded by Jesus and continued by his Apostles. It is the same Church described in the Bible as the Body of Christ and the Bride of Christ (1 Corinthians 12 : 27; Ephesians 5 : 23–25). Throughout its 2000-year history Orthodox Christianity has remained faithful to the teachings and practices passed on from the Apostles and early Church Fathers (2 Thessalonians 2 : 15).
A Timeline of Church History
Articles on Orthodoxy
The following articles on Orthodoxy are from the book, These Truths We Hold - The Holy Orthodox Church: Her Life and Teachings, published by and available from Saint Tikhon's Seminary Press: