Council of Chalcedon (AD 451)
The ancient Church used the title “Orthodox” against the Gnostics, Montanists, Arains and Nestorians to qualify it as the Church having the undefiled faith and their opponents are the heretics. This Church had five main centers at Antioch, Alexandria, Constantinople, Jerusalem and Rome. Today the heads of these Churches are called either Patriarch (Antioch, Constantinople, and Jerusalem) or Pope (Alexandria and Rome).
The ancient Church was divided into two kinds of Christians as a result of the Council of Chalcedon ( AD451).The councils main agenda was to discuss about the Person of Jesus Christ, i.e to say how the divinity and humanity were united in Jesus Christ. The council adopted a doctrinal thesis, which was accepted one group and rejected by the other group. Those who rejected the Council’s decision were called the Non-Caledonians and even today the Churches of Egypt, Syria, Ethiopia, Erithrea, Armenia and India are known by this name; they are also called the oriental Orthodox Christians. On the other side the Christians of the major portion of the Roman Empire accepted the Council of Chalcedon and they were referred to as the Caledonians.