A Western designation for a religious community more accurately known as the “Church of the East,” as the church outside the boundaries of the Roman Empire was designated in late antiquity and subsequently. The polemic epithet “Nestorian,” often applied to the church in historical writing, refers to Nestorius of Constantinople (d. after 451), whose dogmatic position was banned at the Council of Ephesus (431). As reported by ambassadors of the thirteenth century, however, the Church of the East has always rejected the label “Nestorian,” given that this implies heresy. Recent theological analysis confirms that the Church of the East accepted orthodox theology. In the Middle Ages its members were also known as Chaldaeans.
Syriac- (i.e., Aramaic), Persian-, and Greek-speaking Christian communities in the Persian Empire were integrated under a centralized hierarchy from the fifth century onward. The councils of that time formally established the primacy of the metropolitan of Seleucia-Ctesiphon over the Church of the East. The metropolitan, later called catholicos and patriarch, moved to Baghdad after its foundation in the eighth century, and his successors continued to reside there until the Mongol period.
Members of the church kept a considerable role (albeit of declining importance) as bureaucrats in the Muslim administration. They also exerted influence as court physicians and scientists. Under the early Mongols, in whose armies Christians had fought, they retained and even consolidated their position. At that time the Church of the East reached its zenith. Merchants and monks had already carried Christianity as far as South Arabia, Central Asia, India, and China. Although they never convinced either the majority or the rulers of the Asian peoples, the Church of the East was to become the most widespread of all Christian denominations in the Middle Ages. Beside its enterprising merchants and missionaries, the medieval Church of the East is famous above all for its academies and its great scholars, who also were instrumental in the transmission of ancient Greek and oriental philosophy and science. The Syriac language remained in liturgical and scholarly use even after Arabic became dominant. With the rule of Ilkhan Ghazan (1295-1304), decay began, mainly caused by the decline of religious tolerance and the increase of violence toward Christians in Asia.
Studies of the communities of the Church of the East in Outremer are as scarce as are the sources. Until their arrival in the Levant, the Latins had been ignorant of the Church of the East. Itineraries (mainly of the thirteenth century) as well as some legislation, however, show that the Franks took notice of the communities under their rule and lifted the discriminatory poll tax that had been imposed under Muslim rule. But they were legally subordinate to the Latin population. Under the authority of a metropolitan in Damascus and probably a second metropolitan of Jerusalem and Tripoli (mod. Trâblous, Lebanon), the communities lived side by side with the other Eastern Orthodox denominations in the Levant, particularly in Edessa (mod. Şanlıurfa, Turkey), Antioch (mod. Antakya, Turkey), Lebanon, the merchant towns of the coast, and Cyprus. In Cyprus they were placed under the jurisdiction of a Latin bishop. From Tripoli a rhetor named Jacob is known, who taught medicine and rhetoric. Perhaps there were some intellectual groups, enlarged by refugees, which had moved to Outremer during the Mongol invasions in Central Asia and Mesopotamia.
During the twelfth century, the leaderships of the Latin Church and the Church of the East took almost no notice of each other. A century later, information about the enmity between the Muslims and the Mongols was added to older rumors about Prester John, the alleged Christian king in the East. These led to Roman responses to diplomatic initiatives by the Church of the East.
The residences of the mendicant orders in the Levant served as bridgeheads for the Roman missions to the East, such as those of William of Montferrat and Lorenzo of Orte. Contacts with the Church of the East even intensified in the second half of the thirteenth century. Pope Nicholas IV (1288-1292) received a famous Eastern Christian embassy on friendly terms: it was led by a confidant of the catholicos Yahballaha III (1281-1317), the monk Mar Bar Sauma from the area of Beijing. He also visited King Philip IV of France and King Edward I of England. His mission was to communicate to the Western leaders the quest of the Mongols for a new crusade against the Muslims. No sources from the Church of the East corroborate Roman claims that some Eastern metropolitans recognized Roman primacy at that time. The friendly encounters of the thirteenth century should also be interpreted within the context of intensified relations between the Eastern denominations themselves and thus of increased ecumenical activities in the Near East.