Emperor Theodosius (379-395), completed the transformation of the Roman Empire into a Christian empire that had begun under Constantine. The division of the empire in 395 A.D. between the sons Theodosius, Arcadius in the East and Honorius in the West, was in theory an administrative one. In practice, however, the separation was permanent, and each division functioned as an independent state. The political and geographical division between East and West coincided closely with that based on language and culture, and this fact accentuated the separation. The Church also was affected by this separation. This is the origin of Eastern and Western Churches.
The Orthodox Churches
They developed over centuries of estrangement from Rome owing to a great diversity of factors, chief among which must be listed as the differences of theological and spiritual emphasis and political, cultural, and social variations coupled with a fundamentally different ecclesiology, at least in the development and exercise of the organ of the juridical authority.
The first divisions were provoked by the Christological disputes of the fifth century. The Assyrian Church of the East was separated following the condemnation of Nestorianism at the Council of Ephesus in 431 A.D., and the Oriental Orthodox Churches (Armenian, Coptic, Ethiopian, Syrian Orthodox) in the aftermath of the condemnation of Monophysitism by the Council of Chalcedon in 451 A.D. The division between the two great Churches of the Eastern and Western Empire, the Byzantine and Latin, was a process of slow estrangement with several periods of tension (such as the so-called Photian Schism in the mid-ninth century and the Schism of Michael Cerularius in 1054), but was not consummated until the Crusades (1095-1291) and the Orthodox rejection of the Decree of Union of the Council of Florence (1439).
Eastern Catholic Churches: After the heavy losses to the Catholic Church resulting from the heresies of Nestorianism and Monophysitism and from the gradual estrangement that became permanent from the 11th century between the Byzantine-Slav Orthodox and Roman Catholic Churches, there existed no organized Catholic Eastern Churches, with the exception perhaps of Italo-Albanians in Sicily and southern Italy, the Syro-Malabarians in India and the Maronites. Maronite Church is the only Eastern Church which is exclusively Catholic with no Orthodox counterpart. In 1181 the Maronites were reconciled with Rome, and the Armenians in Syria in 1198, but in general the Crusades deepened antagonism between the Orthodox and Catholics, owing principally to the plundering by Crusaders of Orthodox churches and shrines. The capture of Constantinople in 1204 and the establishment of the Latin Empire of Constantinople created a lasting hostility and bitterness. Two large-scale efforts to heal the separation were made, but unsuccessfully, at the Councils of Lyons (1274) and Florence (1439). Decrees of reunion were signed, only to be shortly afterward repudiated by the great majority of the Orthodox clergy and people.
Zealous missionary activities among the peoples of the Near East and Slav countries by Catholic religious orders, especially Jesuits, Dominicans, Franciscans, and Capuchins, bore fruit in the rise of Catholic groups that retained their Eastern rites and customs. When such groups became large enough, Rome setup a hierarchy - even at times a Catholic Patriarchate - corresponding to that of Orthodox Churches.
Until the time of Christ religion and nationality were identified. To accept the Jewish faith meant to accept Jewish culture and life style. There was no distinction between religious expression and cultural expression, between religious identity and ethnic identity. And there wouldn't be until the Church became "Catholic." The passage to "Catholicism" is described in the Acts of the Apostles. When the Church decided to give the Greek Christians their own leaders (Acts 6: 1-7) and to accept converts from paganism without requiring them to become Jews first or to follow the Jewish cultural-religious laws (the Mosaic Law: see Acts 10:1 to 11:18 and 15:1-29), she broke the identification of Christianity with a particular culture or cultural pattern of expression. This is what opened the door for Christ's Church to spread "throughout the whole" (kata-holos), which is what "catholic" means. Its teaching and belief are the same throughout the whole world, but the expression of its belief is embodied differently in the diverse cultural forms of every race, nationality, and ethnic group. This is what we call "different rites" within the Church. There is no such thing, therefore, as a "Roman Catholic Church" which is a contradiction in terms. There is only Roman or Latin rite of the Catholic Church.
In the beginning, Christianity was confined predominantly to urban centers. The inhabitants of rural areas were largely pagan. There were 4 main Christian centers in the empire, Rome, Constantinople, Alexandria, and Antioch. Only Rome was in the west. Two other Christian centers evolved outside the empire - Persia and Armenia. The Church in India (Malabar) came under the control of the Persian Church. All Eastern rites may be classified under five fundamental heads: Alexandrian, Antiochene, Armenian, Persian (Chaldean), and Byzantine (Constantinopolitan). They owe their great diversity not only to differences in rites but also to doctrinal differences.
The Catholic Church can be distinguished into Eastern and Western with their own Canon law codes. Now the Catholic Church consists of 6 rites and 23 distinct Churches (Churches sui juris), the Latin Church or the Western Catholic Church, and the 22 Eastern Catholic Churches each having its own hierarchy in communion with the Pope. Each of these Churches has its own rules, government, liturgy and culturally-influenced way of expressing the faith which all Catholics hold in common.