The divergence of Western and Eastern Christianity was predetermined by the mismatch of socio-historical and cultural-ideological conditions and accelerated by political events such as the first partition of the Empire after Constantine and its final partition after Theodosius. And although this world was United within a single Roman Empire, which was the most important prerequisite for the transformation of Christianity into a world religion, Rome itself accelerated the process of division of this world after the crisis of the third century, when it became apparent the impossibility of preserving the unity of a huge state. From III. and throughout the IV century. division of the Roman Empire was carried out almost continuously with the arrival on the Imperial throne of each new ruler. From 285 (the beginning of Diocletian’s creation of the tetrarchy system) to 395 (the death of Theodosius and the final division of the Roman Empire between his sons), in 110 years, the Roman Empire had only about 20 years of relative unity.

During the period of Christianization in its administrative-territorial and even internal structure of the Christian Church organization adapted to the structure of the state and sought to fit seamlessly into the Roman Empire. Of course, in the absence of political unity, it was extremely difficult to achieve religious unity. Especially since Christianization was accompanied by a sharp religious and political struggle, and therefore disputes about the priority of Episcopal thrones originate in the IV century. The importance of the bishops and the bishoprics headed by them was determined by the political and economic importance of the centers of the Roman Empire in which they carried out their activities. Such centers in the West in the IV century were Rome and Mediolan, and in the East – Constantinople, Antioch and Alexandria. These centers of Church organization were involved in a fierce competition for priority.

The Roman ecclesiastical see has been regarded as the main one since Apostolic times because, above all, of the importance of Rome itself as the center of the entire Mediterranean world. As other bishoprics grew and strengthened during Christianization, the Roman Church sought to maintain its leading position and repeatedly claimed priority in the fourth century. however, the Eastern churches were able to prove their viability and independence. The Council of Constantinople, which met in 381, was exclusively Eastern in its composition. Despite the usual disagreements of Church councils, one of the main results of this Council at that historical moment was the achievement of a certain unity in the Eastern churches. The 3rd Canon of the Council of Constantinople in 381 gave Constantinople its second place in the ecclesiastical world after Rome, although before that the first chair in the East was considered to be that of Alexandria.

In the summer of 382, the Council of the West was held in Rome. Contrary to the decision and letter of the Eastern Council of Rome did not change the previous position of the West and declared that the Roman Church does not depend on the decisions of the Eastern Council and that the 2nd place of Rome gives Alexandria, and the 3rd-Antioch. These decisions meant nothing more than a break between “Western” and “Eastern”.

After the victory of the Archbishop of Constantinople over the Pope of Alexandria at the IV-th Ecumenical Council in 451, he canonically and actually became the first Bishop in the East. His rights are enshrined in the 28th Canon of the Council of Chalcedon. Not content with this, the Archbishop of Constantinople entered into a rivalry with Rome. Relying on the Byzantine Emperor, he did not want to recognize the canonical primacy of the Pope. Strained relations between Rome and Constantinople were under Justinian and Pope Vigilia, and at the end of the sixth century over the naming of the Patriarch of Constantinople, John II, “Ecumenical Patriarch,” and during monothelitic disputes and iconoclastic troubles.

The popes, taking advantage of their geographical and political position, held themselves independent of the Byzantine emperors. Not only did they, like the Eastern patriarchs, disobey the Imperial edicts on matters of faith and Church, but they openly protested against them. It cost the death of Pope Martin I, and less than 100 years later, the popes paid for it with a whole Illyricum. The latter circumstance prompted them to take a heroic step: the popes finally broke off relations with the Byzantine emperors and recognized the Frankish kings as heirs of the Roman Caesars. This happened on Christmas day 800, when Charlemagne was crowned Pope Leo III as Roman Emperor.

Emperor Nicephorus (802-811) forbade the Byzantine Church to communicate with Rome, because, as stated in the letter of Patriarch Nicephorus (806-815) to the Pope: “You (the Romans) themselves separated from the Church.” In the fact of the coronation of Charles by the Pope, the East saw an encroachment on their rights, as the only legitimate romaic rulers-Roman Caesars. Therefore, the subsequent Byzantine emperors could neither forget nor forgive the popes, this fact. At the Council of Constantinople, 869-870. to the solicitation of the papal legates that Bulgaria should remain under the jurisdiction of the Pope, there was a definite refusal, clearly justified: “it is quite indecent that you, who have renounced the Greek Empire and allied yourself with the Franks, should retain your rights to govern in our Kingdom.”