In the context of the crusades, the term Eastern (or Oriental ) churches refers to those Christian communities that were separate from both the Latin (Roman Catholic) Church of the West and the Greek Orthodox Church of the Byzantine Empire. They can be understood as belonging historically to three groups or families.
The Church of the East has been known at various stages in its history as the Nestorian, Chaldaean, or Assyrian Church. Its adherents were found principally in Mesopotamia, Iraq, and Persia, although the church undertook missions as far as Central Asia and China. It had relatively few communities in the Frankish states of Outremer. The Church of the East today is not in full communion with any other church.
The largest group of Eastern churches are often referred to collectively as monophysite churches, from the Greek words for “one nature,” or as non-Chalcedonian churches. They recognized the three church councils of Nicaea, Constantinople, and Ephesos, but rejected the christological definition of the Council of Chalcedon (451), which asserted two natures, divine and human, in Christ. These churches are now in full communion with each other, though they have very different traditions and rites. The Coptic Orthodox Church was mainly confined to Egypt, where it had a substantial number of adherents under Fâtimid, Ayyûbid and Mamlûk rule. Its patriarch resided at Alexandria and later in Cairo. In the medieval period, the churches of Ethiopia and Nubia were also under the authority of the Coptic patriarchate of Alexandria. The Syrian Orthodox Church was represented in Syria, Upper Mesopotamia, and eastern Anatolia, with smaller communities elsewhere; under Frankish rule in the Levant, its adherents were found especially in the county of Edessa, but also in the principality of Antioch and the kingdom of Jerusalem. The church was mainly organized by Jacob Baradai in the sixth century, and thus has also been known as the Jacobite Church; its preferred modern designation is the Syriac Orthodox Church. Its patriarch, originally based at Antioch, has changed his place of residence several times, and is now resident in Damascus. The Armenian Orthodox (or Apostolic) Church is the national church of the Armenians, who were the first people to adopt Christianity as a state religion. In the Middle Ages it was organized under four main centres: the catholicosate of Echmiadzin in Greater Armenia (302), the patriarchate of Jerusalem (1311), the catholicosate of Cilicia (1441), and the patriarchate of Constantinople (1461).
The third family of churches comprises those that recognize the Council of Chalcedon. In the period of the crusades, these were principally the Maronite Church, which originated in the struggle concerning the monothelete doctrine, and the Georgian Orthodox Church, which around 600 separated from the Armenian church and accepted the Council of Chalcedon.
From the time of the Frankish conquest of Outremer, the Latin Church made efforts to achieve union with the Eastern churches, although the only successful union in the medieval period was with the Maronite Church. Unions with other Eastern churches occurred much later (between the seventeenth and nineteenth centuries). In all these cases only parts of the respective church entered into union with Rome, resulting in schisms, so that today there are various Eastern Rite Catholic churches in existence alongside the Eastern churches. Since the nineteenth century, Protestant branches of some of the Eastern Orthodox churches have come into existence as a result of missionary activity, but their numbers are very small.