The Orthodox Church
The Orthodox Church, as it is known today, is the Apostolic Successor to the original patriarchates of the East, at Antioch, Jerusalem, Constantinople, and Alexandria. For a millennium and more these four Patriarchates existed alongside the Patriarchate of Rome, chronologically second in Her order of foundation, but first amongst equals in Her place of honour due to Rome's exalted place as the Imperial City. These Patriarchates were the fruit of Christ's great commission, when ' Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen.'.
His disciples heeded this commission, as the first bishops, and founded various Episcopal Sees. As time went on, and the faith grew throughout the world, more Patriarchates and Autonomous Churches were formed out of those already existing, bound together in the unity of an unshaken and unchanged faith.
Sadly, the Church of Rome, possibly feeling slighted by Constantinople, which had replaced Rome in primacy as the Episcopal See of the current Imperial City, began to take matters into Her own hands, and, little by little, unilaterally began to introduce new ideas of her own, both in the realms of Theology and in Ecclesiology. This became, in the 11th Century, a matter too much for the Eastern Patriarchs to bear, and communion was, over the course of the next couple centuries, slowly but systematically broken.
The Orthodox Churches continued, often under great persecution, however, and exist today in glory, proclaiming the true Faith of the Fathers, and preserving that faith without development of doctrine. Truly, Christ's marvellous promise 'lo, I am with you always, even unto the end of the world', has been a promise fulfilled.