The Pentarchy consisted of the five ancient patriarchates of the undivided Church of the first millennium of her history, including the Churches of Rome, Constantinople, Alexandria, Antioch, and Jerusalem.
These major centers of early Christianity, founded by the apostles, were looked to by their respective regions as leaders in Church life, and eventually their bishops came to be regarded as the primates of their areas. The members of the Pentarchy all participated in some form in the first eight Ecumenical Councils, from 325 to 880. Their relationship with each other, despite various periods of rivalry and dispute, was generally in terms of fraternal equality and conciliarity.
After the Ascension, the apostles dispersed to preach Christianity to the world. They each founded different patriarchates. Some of the most prominent disciples of Jesus founded the patriarchates that made up the Pentarchy.
After the seventh-century Arab conquests and the Byzantine loss of the Rome-Ravenna corridor, only Constantinople's patriarchate remained securely within the capital of the Roman Empire—the Pope at Rome was independent (see Gregory the Great), Jerusalem and Alexandria were under Muslim rule, and Antioch was on the front lines of hundreds of years of recurring border warfare between the Byzantine Empire and the Arab Caliphate. Also during the Middle Ages, the center of gravity of Christendom had shifted northward, and the majority of Christians in Muslim-ruled Egypt and Syria were Non-Chalcedonians who refused to recognize the authority of either Rome or Constantinople. Together, these historical-political changes meant that the original ideal of five great co-operating centers of administration of the whole Christian Church grew ever more remote from practical reality.