Many in the West see Christianity as a European religion, but its roots reach far back to the ancient Near East.
The first nation to officially become Christian was not the Roman empire under Constantine, as is generally thought, but the ancient kingdom of Armenia (comprising much of modern eastern Turkey), whose king, Tiridates, was converted by Gregory the Illuminator in 301.
Likewise, one of the largest and most important ancient Near Eastern branches of Christianity was the "Church of the East," more commonly known as the Nestorian or Assyrian Church. For 1,500 years, Nestorians were the predominant form of Christianity in Iraq and Iran, and most of Asia.
According to Nestorian legends (recorded in the third century Acts of Thomas), within a few decades after the death of Jesus Christ, Christianity was introduced into Iraq by Thomas the apostle and Thaddaeus, one of the Seventy.
In the following two centuries, missionaries formed Christian communities throughout Iraq, Arabia, Iran and the west coast of India.
Although Christianity never became the predominant religion in any of these regions, it has been estimated that by the 4th century, 10 percent of the population of Iran and Iraq were Christians — about the same as the percentage of Christians in the Roman empire at that time.
However, whereas in Rome, the emperor Constantine converted to Christianity, thereby facilitating the rapid conversion of his empire, in the east no Persian emperor ever converted; instead they remained faithful to their ancient Zoroastrian heritage.
Although initially in communion with the Christians within the Roman empire, doctrinal controversy caused a schism between the Nestorians and western Christians in the 5th century.
The name "Nestorian" derives from the Syrian theologian and monk Nestorius (386-451). His fame as a scholar and preacher eventually reached the Roman emperor Theodosius, who, in 428, appointed him as patriarch of Constantinople — the highest Christian office of the time, more significant than the pope of Rome.
Nestorius's brief reign as patriarch was filled with controversy, centering on questions about the divine nature of Christ. He rejected the use of the term "Mother of God" (theotokos) to refer to the Virgin Mary, since he believed Mary was the mother only of the human Jesus, not the divine Christ.
He was condemned as a heretic by the Roman pope Celestine in 430, but Nestorius, as patriarch of Constantinople, refused to recognize the pope's authority. A general council of the bishops of the church was summoned to resolve the controversy; meeting at Ephesus in 431 they deposed Nestorius. He was banished to upper Egypt, where he died around 451.
From this period Nestorian Christians — those who rejected the Council of Ephesus and followed the theological interpretations of Nestorius —became increasingly distinct from the West, developing an independent theology, liturgy and hierarchy. The fundamental Christological idea of the Nestorians was that there are two separate "persons" in Christ, one human and one divine; Catholic and Greek Orthodox Christians, following the formulations of the Council of Ephesus, generally believed that Christ was a single "person": simultaneously fully human and fully divine. These irreconcilable theological differences culminated in the formation of a distinct Nestorian denomination.
For the next thousand years, Nestorians were the major form of Christianity east of the Euphrates River. One of the great achievements of the Nestorians was their missionary efforts throughout Asia. In the early 7th century — when the Anglo-Saxons of England were first converting to Christianity, and when their Viking cousins were still three centuries from conversion — a Nestorian missionary named Abraham reached China. In 635 he preached to the Chinese emperor, was granted imperial recognition, and established a Nestorian Christian mission in China which flourished for centuries. (Visitors to Xian China's "Forest of Stelae" Museum can still see a a large stone inscription written in Chinese and Syriac describing this grant of imperial recognition to Nestorian Christians.)
Nestorian missionaries also preached among the nomads of Central Asia, converting several tribes to Christianity. Rumors of the existence of these Christian nomads reached Europe in the form of the legends of Prester John. In 1287, the Mongol emperor Kublai Khan dispatched a Mongolian Nestorian monk named Sauma as his ambassador from Beijing to the pope at Rome; the diaries of his journey to Italy survive and make fascinating reading.
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In India, on the western coast of Malabar, communities of Nestorian Christians developed, tracing their origins back to the preaching of Thomas the apostle.
Numbering several hundred thousand today, small communities of Nestorian Christians still survive in Iraq, Iran and India. Emigrant Nestorian communities can also be found in England and in the Chicago area.
Further Reading: "Christoph Baumer, The Church of the East: An Illustrated History of Assyrian Christianity," (Tauris, 2006).