Christianity arrived in Iran during the Parthian (Ashkanian) period. In the book of 'Acts of Apostles' (chapter II, V.9) first century AD, it is mentioned that on "the Day of Pentecost (part of harvest festival observed by early Christians) there were at Jerusalem "Parthians, and Medes, and Elamites, and inhabitants of Mesopotamia". Early Christian records mention that Peter and Thomas preached the Gospel to the Parthians and men such as Thaddaeus, Bartholomew, and Addeus evangelized the races of Mesopotamia and Persia, and that Mari, a noble Persian convert, succeeded Addeus in the government of the Persian Christian communities. The bishops, AbrÍs, Abraham, Jacob, Ahadabuhi, Tomarsa, Shahlufa, and finally bishop Papa succeeded him (end of the third century). Syriac documents also indicate that towards the beginning of the third century the Christians in the Persian territories had some three hundred and sixty churches, and many martyrs.
Arbela, fifty miles east of river Tigris (Dejleh), the capital of Adiabene a small Persian border kingdom was the earliest center of Christianity in Iran (present day Iraq). There was a large concentration of Jews in Arbela and in Nisibis in eastern Mesopotamia and while some Jews were instrumental in spreading Christianity others opposed the new faith. The first century Jewish historian, Josephus mentions that a king of Adiabene accepted Judaism about AD 36. Such a conversion made Arbela a natural center for Jewish Christian mission at an early date. Nisibis another major city of the area was also the seat of a Jewish Academy of learning. Christianity spread in both Villages and cities and by the end of the Parthian period (AD 225), Christian communities were settled all the way from Edessa, an important missionary center, to Afghanistan. The Chronicles of Arbela report that by this time there were already more than twenty bishops in Persia and Christians had already penetrated Arabia and Central Asia.
Parthian Kings were tolerant of other religions and Christianity kept slowly but steadily advancing in various parts of the empire. At the time of the persecution of Christians in Rome many sought refuge in Iran and were given protection by the Iranian rulers. Though thousands of Persians embraced Christianity, Persia remained Zoroastrian with many adhering to the Cult of Mithra. There never arose an indigenous Persian Church, worshipping in the Persian language. The Persian Church was of Syrian origin, traditions and tendencies and for about three centuries, regarded Antioch (in Syria) as the center of its faith and the seat of authority.
With Sassanian (A.D. 226-641), Christianity (and other religions) suffered resentment. Its chief opponents were the Zoroastrian Magi and priestly schools, as well as some Jews. When the Roman emperor Constantine made Christianity the state religion in Rome (AD 312) and himself the sovereign of all Christians, the new fate became associated with Iran's archenemy. Conversion of Armenians into Christianly and defection of some Armenian army units to Rome made the matters worse. Religious and national feelings were united and paved the ground for future persecutions that continued in Persia for a century after they had ceased in Rome, where they started in the first place.
The Sassanian kings in general championed Zoroastrianism, and though some did not mind Christianity, the national feeling always clung to the ancient creed. Nevertheless Christianity kept steadily growing partly due to deportation of several hundred thousand Christian inhabitants of Roman Syria, Cilicia and Cappadocia by Shapour I (240-270AD). The deportees wee settled in Mesopotamia, Persis (Pars) and Parthia. The decision was based on economic and demographic reasons but unintentionally promoted the spread of the new faith. New cities and settlements in fertile but sparsely populated regions such as Khuzistan and Meshan were built. Many Christians were employed in big construction projects and had a large number of skilled workers and craftsmen among them. The city soon became a significant cultural and educational center with the famous library and University of Jundaishapour, home to scholars from all over including many Christian and Jewish scholars. It also became the center of silk production in Iran with many Christians involving in every aspect of silk production, management and marketing.
This period of peace and prosperity for the Christian community lasted until the reign of Bahram II (276-293AD). First persecutions included that of Bahram's Christian concubine Candida, one of the first Persian Martyrs. The persecutions were supported and even promoted by the powerful high priest Kirdir who in one inscription declares how Ahriman and the idols suffered great blows and continues as follows: "and the Jews (Yahud), Buddists (Shaman), Hindus (Brahman), Nazarenes (Nasara), Christians (Kristiyan), Baptists (Makdag) and Manicheans (Zandik) were smashed in the empire, their idols destroyed, and the habitations of the idols annihilated and turned into abodes and seats of the gods".
But these persecutions remained exceptions compared to the fourth century when systematic harassment of Christians began. Originally Christianity had spread among the Jews and the Syrians. But by the beginning of the fourth century, Persians in increasing numbers were attracted to Christianity. For such converts, even during peaceful times, membership in the church could mean loss of family, property, civil rights and even death. Some persecutions under Shapour II (309-379AD) were as horrid as those administered by the Roman Emperor Diocletian who used to burn or feed the Christians alive to wild beasts, or have them killed publicly at the games by the gladiators.
Towards the beginning of the fourth century the head of the Persian Church selected the city of Seleucia-Ctesiphon, the capital of the Persian Empire, as his center of authority (Ctesiphon metropolitan). Under his jurisdiction were several bishops, one of them, Yohannan bar Maryam of Arebela was present at the very important Council of Nicaea (325 AD) in Rome. In 340 or 341 AD, the new metropolitan (Archbishop) of Ctesiphon, Shem'on (Simeon) bar Sabba'e, was urged by Shapour II, to collect a special tax from the Christians to finance the costs of war against Rome. His refusal was the prelude to the systematic persecution of Christians. In the Martyrology of Simeon, Shapour is quoted of accusing the bishop of having political motives for his policies. While the Persian sage, Aphrahat, the most important intellectual representative of Christianity in Iran at the time in his Demonstrations compares Constantine with good and the proud Shapour with forces of evil.
Aphrahat was an Assyrian born in northern Mesopotamia in the region of Adiabene and was a monk, probably a bishop. His only surviving work Demonstration contains 23 treatises, which he wrote between AD 337 and AD 345. The first ten chapters of Demonstrations deal with ten specific aspects of Christian life and doctrine such as faith, fasting, prayer and humbleness. In this he displays a very simple faith, firmly centered on the Scriptures. For him a "Christian life must be a life of unrelenting warfare between believers and the devil. The most dangerous instrument of satanic temptation is a woman; the safest path for man, therefore, is to renounce the love of a woman, and live alone for Christ. As for women, their highest calling is to espouse virginity and thus rob the devil of his tool for temptation. Since it was not possible for all to remain celibate, Christians may marry, but if they do, it might be best to marry before baptism". In his address to the monks he recommends that "if a monk desires, that a woman bound by celibacy, should dwell with him, it would be better for both parties to marry and live openly together" (Demonstrations VI.4). His ideas were picked up over a century later when the church had to make a decision about celibate clergy.
Shapour was not the only enemy; in the Chronicles of Arbela Christians blame Magi, Jews and Manicheans for promoting hatred against Christians and calling them Roman spies. In fact some Zoroastrian authorities such as mogbed and rad (titles in priestly hierarchy) are named for being directly involved in interrogating and convicting Christians at times of persecutions (Syriac Acts of Martyrs). Some Christian accounts of martyrdoms show anti-Jewish tendency, and the same is true of some writings of the Eastern Church fathers. Weather those Christians had political motives or not needs more research, however surviving literature indicates that they indeed regarded their faith as superior. Their world was not divided between Romans and Iranians but between 'people of God' and the 'outsiders' or 'non believers'. In their literature they identify themselves as 'pure ones', 'just ones' or 'people of God'. Distinctions are made between ethnic Christians, nasraye and deported ones and their descendants called Krestyane. They also referred to themselves as misihaye (those who believe in Messiah (Massih).
Shapour's peace treaty with Emperor Jovian halted the persecutions for a while (AD363). By this treaty, Mesopotamia and Armenia came under the control of Persia. In AD 409, the Persian king Yazdegard I, by an edict of toleration brought an end, for the time being, to the persecution of Christians. He had a Jewish wife and was well disposed towards both Judaism and Christianity and in fact was called the 'Christian King' by some. The edict allowed Christians to publicly worship and to build churches. The peace helped the Christian community to re-organize its life. Tensions eased further when Iranian Christians created their own ecclesiastical organizations with its own hierarchy and eventually became independent from the Western Church.
Though Rome and Constantinople were the centers of the so-called 'Orthodox Christianity', many Christian groups particularly in Mesopotamia opposed their policies and doctrines. In 410, a meeting of Christians was held at the Persian capital under the presidency of Mar Isaac, the bishop of Ctesiphon. An independent new Church was announced and the leader (metropolitan) was called 'Catholicos-Patriarch'. The council confirmed Mar Isaac as the first Catholicos and Archbishop of all the Orient.
The Synod (Ecclesiastical/Church council) also declared its adherence to the decision of the Council of Nicea in Rome and subscribed to the Nicene Creed. Though the church was not fully independent from Rome as yet, Yazdegerd approved of the organization of the Persian church on this basis and issued an edict giving recognition to the Catholicos as the head of the Persian church. Christians in Iran received a definite standing among the population, with freedom to manage their own affairs, but answerable to the state authorities through the Catholicos who became a civil as well as a religious head. The decree also dictated that the election of a Catholicos had to be approved by the king and he became king's nominee.
Early in Yazdegerd's reign Maruthas, a Mesopotamia bishop represented the Roman Emperor at the Persian Court. He was instrumental in re-organizing the Persian Church and spreading Christianity further in Iran and Nisibis became a strong Christian center. Later in the reign of Yazdegerd, the Persian bishop, Abdas of Susa destroyed a Zoroastrian temple in the city; the king ordered the bishop to restore the building at his own expense. Abdas refused and the result was the order by the king to destroy all churches. Before long the destruction of churches developed into a general persecution, in which Abdas was one of the first martyrs. When Yazdegerd died in 420, and was succeeded by his son Bahram V, the persecution continued, and large numbers of Christians fled across the frontier into Roman territory. Bahram demanded the surrender of the Christian fugitives, and once again war was declared against Rome in 422. Although the latter half of the fourth and the beginning of the fifth century was a period of conflict in the Eastern provinces, the period was also a time of expansion for the Christian Church and of literary activity. This literary and ecclesiastical development led to the formation of a Syriac literature in Persia (Syriac being the liturgical language of the Persian Church), and ultimately of a Christian Persian literature. By 420 there were 5 metropolitans including two at Merv and Heart and bishop Dadyeshu was elected Catholicos. He was imprisoned a year later and internal divisions and disputes were intensifying at the time amongst different Christian denominations.
During the rule of Bahram V (421-438) the third synod of the church introduced a radical change. The Synod of Dadyeshu met in 424 under the presidency of Mar Dadyeshu. The first synod of Isaac in 410 had decided that the Catholicos of Seleucia-Ctesiphon be supreme among the bishops of the East. The Synod of Dadyeshu decided that the Catholicos should be the sole head of the Persian church with no one above him. In particular it was laid down that "easterners shall not complain of their Patriarch to the western Patriarchs; every case that cannot be settled by him shall await the tribunal of Christ."
This meant that their Catholicos was answerable to God only and not to Rome, Antioch, Alexandria or Constantinople. Six metropolitans and thirty conventional bishops from all over Persia elected Dadyeshu and he became the first Catholicos equal in rank and authority to any western Patriarchate. This gave the Iranian church the privilege of independent administration and freedom from outside jurisdiction. For a while King Yezdegerd II (439-457AD) welcomed the move and sent the Patriarch of the Persian Church on a mission to meet the Roman Emperor.
The king took a particular interest in the question of religion and studied all religions practiced in Iran. But he remained a zealous Zoroastrian and at the end started persecuting both Christians and Jews. He tried to convert Armenians back into Zoroastrianism; he was defeated once, won again and took hostage the leaders of the Armenian Church and leading members of the local aristocratic families by carrying them off to Iran. The next successor Peroz (459-84) faced many disasters and wars and ended up a hostage. He persecuted the Jews and watched the Christian community going through internal conflict and doctrinal divisions.
In 486 the church made a decision that went against the radical ascetic tendency of the East and against the canon laws of the West. It rejected celibacy and affirmed the rights of all Christians to marry including ordained priests or even bishops. The texts mention social and cultural factors for this verdict. But the state also pressured the church to change its stand on celibate clergy. Zoroastrians held the unmarried clergy in contempt and considered celibacy as a cause of weakness in the empire. The virtue of virginity irritated them and there are accounts of nuns forced out of monasteries to be married and were put to death if refused. This movement against the enforced celibacy of the clergy did not last and the decision was reversed in the sixth century.
Between 450 and 500 the Nestorians, followers of Nestorius the patriarch of Constantinople who created his own brand of Christianity were persecuted in the Roman Empire. They fled to Persia and received protection. Nestorianism had been rejected at a meeting of Christians from all over at 431 in Ephesus (Turkey) and their bishops were forced to flee to Iran. From 488 during the reign of Qubad, the whole Persian Church adopted Nestorianism at the synod of Jundaishapour (Syrian Beth Lapat) and henceforth the Catholicos of Seleucia became the patriarch of the Nestorian Church of Persia, Syria, China, and India. Nestorians believed in the doctrine of the two natures of Christ (human and divine) as opposed to Monophysite's believe in one nature only. The Nestorian doctrine was popular in the Persian border districts, in the 'Persian School' of Edessa and it was also a way to eliminate the suspicion of conspiracy with the Romans.
The 'Persian School" was closed and transferred from Edessa now a Monophysite stronghold to Nisibis and became very famous. The first rector was the leprous Narses (Narsai) a prolific writer he enjoyed immense reputation. He was a great poet and his gift for language made him a master of the Syriac idioms. His scholarship helped the church to be built on strong biblical and theological foundations and was later honored by the title Rabban the Great'. The central aspect of the school was its spiritual discipline, Bible study and missionary work.
This university consisted of a single college, with the regular life of a monastery. Its rules are still preserved. At one time it had more than 800 students. The fame of this theological seminary was so great that it inspired the Italian Pope to establish the Cassiodorus's monastery at Vivarium. Other less important schools existed at Seleucia and elsewhere, some in small towns and another major one at Jundaishapour. The most colorful Christian personality of the period was Barsauma, who fought for the success of the Nestorian confession, founded the new school in Nisibis and was very active politically. He also rebelled against the leader of the Christian community Catholicos Babuwai.
Khosro Anoshirvan's (531-79) wars against Byzantium (540-545) and Emperor Heraclius's victories once more prompted persecutions but peace was resumed afterwards. The king once again guaranteed their freedom of worship and many celebrated Christians such as the philosopher Paul the Persian and members of the famous learned family Bukhtishu joined the royal court and Jundaishapour University. His successor Hormizd IV (570-90) furthermore supported Christians. His mother was the Byzantine princess Maria a Christian and his support created a backlash amongst the Zoroastrian clergy with violent results against Christians. Khosro II, Parviz (579-90) regained his thrown from Bahram Chobeen with help from his father-in law Emperor Mauritius and remained loyal to Christians. He paid honor to Virgin Mary and to a number of saints popular among the Syrians. His wife remained a devoted Jacobite and was immortalized in Persian literature as Queen Maryam in the love story "Khosro and Sheereen". However Khosro Parviz soon turned against Christians when new wars broke out once again.
Khosro Parviz sacked Jerusalem in 610, his Syrian troops looted the city for 3 days, massacred thousands of Christians and religious relics including a piece of the true cross (the one Jesus died on) were carried off to Iran. The cross itself became a center of dispute amongst Byzantium and Iran and eventually was returned as part of a peace treaty. The official teaching of the Nestorian Church at the time of Khosro II is preserved in the treatise "De Unione" composed by the energetic monk Babai the Great.
In the next century the Persian Church kept steadily increasing with a hierarchy of 230 bishops. Christians were scattered over Assyria, Babylonia, Chaldea, Arabia, Media, Khorasan and Persia proper, Turkestan, Merv and both shores of the Persian Gulf. The figure, 'Catholicos of Seleucia' became a powerful entity and the extent of his jurisdiction rivaled the Byzantine patriarchs. On the whole Christian missionaries were successful amongst all groups including high-ranking Iranians. There are accounts of Christians among the landlord classes in Mosul and the surrounding mountains. Khosro III (630) was killed in an insurrection headed by a Christian whose father had been the chief financial officer of the realm. Some of the patriarchs of the Nestorian Church were converts, or sons of converts, from magi priesthood.
Monasteries were introduced in Mesopotamia by monks from Egypt in fourth century and spread quickly. Accounts by Mar Awgin relates that his monastery near Nisibis contained three hundred and fifty monks, while seventy-two of his disciples established each a monastery. Their numbers must have been very high, in addition to the numerous monasteries in Mesopotamia and the regions north of the Tigris, there were scattered monasteries in Persia and Armenia. Besides the cenobites, living in large communities, there were numerous solitaries living in caves or rude huts. Christian mysticism spread through monasteries and greatly influenced Islamic mysticism that emerged in the area after the Muslim conquest .
While numerous, however, the Iranian Christians were not organized into a national church. They differed from the Nestorians farther west but not enough to gain ecclesiastical independence from Nestorianism. Syriac was the ecclesiastical and theological language and even in Persia proper little Christian literature was produced in Persian and the Scriptures had not been translated into Persian either. A few works were produced in Middle Persian mainly to clarify the legal status of Christians In Iran. The Corpus Iuris by the Metropolitan Mar Ishobukht, dating from 8th century is one that has survived in Syriac translation. Other Christian legal books survived in Syriac are a text by the Metropolitan Mar Simeon and one written under Mar Aba in the reign of Khosro I, Anoshirvan (531-539). Mar Aba was a convert from Zoroastrianism, and had studied Greek at Nisibis and Edessa and intended to prepare and publish a new version of the Old Testament, a task he did not finish. He died in prison and his successor was put to death. In 567 Ezechiel, a disciple of Mar Aba, was appointed Catholicos of Seleucia, under whom lived Bodh the periodeutes, the translator into Syriac of the Indian tales "Kalilah and Dimnah". The Indian literature was made popular in Iran through Jundaishapour University's translations of Indian texts.
With the growth of church many differences arose between different confessions, and this probably is one more reason why the church did not evolve into a national Iranian church. The differences, conflicts and rivalries were significant and created many problems amongst the Christians and eventually helped their downfall and the total defeat of the Christian Church after the Muslim and Mongol conquests both in Iran and outside. Matters were further complicated when some converted from the Church of the East to the Roman Catholic denomination. This group was called Chaldeans who rejected Nestorianism at the AD 451 Council of Chalcedon near Constantinople. They adhered to their separate Patriarch in Syria and created a massive rift between Constantinople, Alexandria, Antioch and Rome. Supported by Byzantine Emperors they started persecuting other Christian sects and took control of many local churches.
Armenian and Assyrian churches made the matters worse. Owing to the war with Persia, the Armenian Church did not have a delegate at the Chalcedon council nevertheless they took side against Nestorians. The Nestorians of Persia were quarreling with the Orthodox Church of Persia, which was in communion with the Church of Armenia and asked for their help. Armenians responded and their Catholicos Babgen called a meeting not only of his own bishops but also those of the neighboring Christian countries of Georgia and Caucasian Albania.
They assembled at the headquarters of the Armenian Church in Dvin in the year 506. After long deliberations they officially proclaimed their adherence to the Ecumenical Council of Ephesus and rejected both Nestorians and Chaldeans. The result was the unintentional separation of the Armenian Church from the rest of Christendom, that is, of Greco-Roman Christianity.
Assyrian Christians were also divided into different confessions. Assyrians (Assori) are one of the oldest surviving Christian groups and currently there are around 550,000 left and almost half still live in Iraq. They are descendents of the ancient Assyrians, a major Mesopotamian Empire from 2000BC, destroyed in 612 BC by the Babylonians and Medes. After this collapse the remnant of the Empire was called Urhai and later Edessa. Many Assyrians fled to the secluded mountains of Kurdistan; some settled in Urumiah in northwestern Persia, and others scattered throughout Asia Minor. Presently they occupy the mountains and plains of southern Turkey, Northern and northwestern Iran and many have emigrated to Europe and North America. They speak various dialects of Aramaic a Semitic language and have kept Chaldean as their religious language. According to their chronicles, they embraced Christianity in the first century A.D. Up till the 16th century, prior to penetration of the Jesuit and later Protestant missions in the Middle East, the Assyrians belonged to two ancient Christian denominations: The Church of the East and The Syrian Orthodox Church, popularly known as Jacobite. The split into two different denominations occurred in the 5th century A.D. and appears to have been politically motivated to secure a measure of safety for the Assyrian minority which was caught between two rival empires: Persian (the locus of the Church of the East) and Roman (the locus of the Syrian Orthodox Church). During Sassanian era majority of the Assyrians in Iran adopted Nestorianism and this created a division between them and the Jacobite Assyrians.
Christianity spread in Iran and affected other sects such as Manicheans (Manavi) and persecutions eventually ended. Despite all improvements, Christians of Iran denied the Sassanian their support once the Arabs attacked the Empire. The motive might have been a feeling of affinity with Christian Arab tribes. However once conquered, Christians like Jews became second-class citizens.