East-West Schism
There were many bishops in the church, and in the East the Pope was considered the "first among equals", but Rome claimed that the Pope was the primary Apostolic see, being that of Peter. At first the Eastern Orthodox did not mind the Pope's claim for power over the West - as long as his power stayed in the West. But it came to pass that the Pope decided he also had power over Eastern Christendom as well and he tried to enforce his power on the eastern Patriarchates.  A letter written in 865 by Pope Nicholas claims the Pope's power extends "over all the earth, that is, over every church".  Pope Nicholas also went beyond his powers as stated in Canon 111 of the Council of Sardica (343) when he overturned a verdict and ordered a retrial on a matter concerning Photius and St. Ignatius, two Patriarchs of Constantinople. This was as a result of petitions to Rome by backers of Ignatius.  However, it should be noted that not all the Popes after Nicholas were as extreme - that is, until Pope Leo IX.  When, in 1053 Cerularius attempted friendly relations with Pope Leo IX after a disagreement, a bull (religious legal document) of excommunication (expulsion from the Church) was brought to Constantinople. Each Church excommunicated the other.