Christianity as the Roman state religion
In the year before the First Council of Constantinople in 381, Nicene Christianity became the official religion of the Roman Empire when Theodosius I, emperor of the East, Gratian, emperor of the West, and Gratian's junior co-ruler Valentinian II issued the Edict of Thessalonica in 380, which recognized the catholic orthodoxy, as defined by the Council of Nicea, as the Roman Empire's state religion. Historians refer to this state-sponsored church using a variety of terms: the catholic church, the orthodox church, the imperial church, the Roman church, or the Byzantine church, with some also used for wider communions extending beyond the Roman Empire. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church all claim to stand in continuity from the Nicene Christian church to which Theodosius granted recognition. Political differences between the Eastern Roman Empire and the Persian Sassanid Empire led to the separation of the Church of the East in 424. A doctrinal split within the imperial church led to the independence of early Oriental Orthodoxy, while the fall of the Western Roman Empire initiated the gradual separation between Eastern and Western Christianity, culminating in the East-West schism of 1054. The Western Church evolved into the Latin Catholic Church while the Eastern Church that remained under the patronage of the Eastern empire and evolved into the Greek Orthodox Church.
Earlier in the 4th century, following the Diocletianic Persecution of 303–313 and the Donatist controversy that arose in consequence, Constantine the Great had convened councils of bishops to define the orthodoxy of the Christian faith and to expand on earlier Christian councils. A series of ecumenical councils convened by successive Roman emperors met during the 4th and the 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the theological and christological doctrines of Arianism, Nestorianism, Miaphysitism, and Dyophysitism. In the 5th century, the Western Roman Empire decayed as a polity; invaders sacked Rome in 410 and in 455, and Odoacer, an Arian barbarian warlord, forced Romulus Augustus, the last nominal Western Emperor, to abdicate in 476. However, apart from the aforementioned schisms, the church as an institution persisted in communion, if not without tension, between the East and West. In the 6th century, the Byzantine armies of the Byzantine Emperor Justinian I recovered Italy and other regions of the Western Mediterranean shore. The Byzantine Empire soon lost most of these gains, but it held Rome, as part of the Exarchate of Ravenna, until 751, a period known in church history as the Byzantine Papacy. The early Muslim conquests of the 7th–9th centuries would begin a process of converting most of the then-Christian world in the Levant, Middle East, North Africa, regions of Southern Italy and the Iberian Peninsula to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Christian missionary activity directed from the capital of Constantinople did not lead to a lasting expansion of the formal link between the church and the Byzantine emperor, since areas outside the Byzantine Empire's political and military control set up their own distinct churches, as in the case of Bulgaria in 919.
Justinian I, who became emperor in 527, recognized the patriarchs of Rome, Constantinople, Alexandria, Antioch, and Jerusalem as the supreme authorities in the state-sponsored Chalcedonian church apparatus. However, Justinian claimed "the right and duty of regulating by his laws the minutest details of worship and discipline, and also of dictating the theological opinions to be held in the Church".
In Justinian's day, the Christian church was not entirely under the emperor's control even in the East: the Oriental Orthodox Churches had seceded, having rejected the Council of Chalcedon in 451, and called the adherents of the imperially-recognized church "Melkites", from Syriac malkâniya. In Western Europe, Christianity was mostly subject to the laws and customs of nations that owed no allegiance to the emperor in Constantinople. While Eastern-born popes appointed or at least confirmed by the emperor continued to be loyal to him as their political lord, they refused to accept his authority in religious matters, or the authority of such a council as the imperially convoked Council of Hieria of 754. Pope Gregory III was the last Bishop of Rome to ask the Byzantine ruler to ratify his election. With the crowning of Charlemagne by Pope Leo III on 25 December 800 as Imperator Romanorum, the political split between East and West became irrevocable. Spiritually, Chalcedonian Christianity persisted, at least in theory, as a unified entity until the Great Schism and its formal division with the mutual excommunication in 1054 of Rome and Constantinople. The empire finally collapsed with the Fall of Constantinople to the Islamic Ottoman Turks in 1453.
The obliteration of the empire's boundaries by Germanic peoples and an outburst of missionary activity among these peoples, who had no direct links with the empire, and among Pictic and Celtic peoples who had never been part of the Roman Empire, fostered the idea of a universal church free from association with a particular state. On the contrary, "in the East Roman or Byzantine view, when the Roman Empire became Christian, the perfect world order willed by God had been achieved: one universal empire was sovereign, and coterminous with it was the one universal church"; and the church came, by the time of the demise of the Byzantine Empire in 1453, to merge psychologically with it to the extent that its bishops had difficulty in thinking of Nicene Christianity without an emperor.
The legacy of the idea of a universal church carries on in today's Catholic Church, Eastern Orthodox Church, Oriental Orthodox Churches, and the Church of the East. Many other churches, such as the Anglican Communion, claim succession to this universal church.
History
Early Christianity in relation to the state
Before the end of the 1st century, the Roman authorities recognized Christianity as a separate religion from Judaism. The distinction, perhaps already made in practice at the time of the Great Fire of Rome in the year 64, was given official status by the emperor Nerva around the year 98 by granting Christians exemption from paying the Fiscus Iudaicus, the annual tax upon the Jews. Pliny the Younger, when propraetor in Bithynia in 103, assumes in his letters to Trajan that because Christians do not pay the tax, they are not Jews.Since paying taxes had been one of the ways that Jews demonstrated their goodwill and loyalty toward the empire, Christians had to negotiate their own alternatives to participating in the imperial cult. Their refusal to worship the Roman gods or to pay homage to the emperor as divine resulted at times in persecution and martyrdom. Church Father Tertullian, for instance, attempted to argue that Christianity was not inherently treasonous, and that Christians could offer their own form of prayer for the well-being of the emperor.
Christianity spread especially in the eastern parts of the empire and beyond its border; in the west it was at first relatively limited, but significant Christian communities emerged in Rome, Carthage, and other urban centers, becoming by the end of the 3rd century, the dominant faith in some of them. Christians accounted for approximately 10% of the Roman population by 300, according to some estimates. Christianity then rapidly grew in the 4th century - Rodney Stark estimated that Christians accounted for 56.5% of the Roman population by 350. According to Will Durant, the Christian Church prevailed over paganism because it offered a much more attractive doctrine and because the church leaders addressed human needs better than their rivals.
In 301, the Kingdom of Armenia, nominally a Roman client kingdom but ruled by a Parthian dynasty, became the first nation to adopt Christianity as its state religion, with the possible exception of Osroene in 201.
Establishment and early controversies
| Communion | Major churches | Primary centers |
| Chalcedonian Christianity | Catholic/Orthodox Church Georgian Church Church of Carthage Church of Cyprus Maronite Church | Rome, Alexandria, Antioch, Constantinople, Georgian Kingdoms Carthage Province of Cyprus Phoenicia |
| Nestorianism | Persian church | Syria, India, China Sassanid Empire |
| Miaphysitism | Armenian Church Coptic Church Syriac Church Ethiopian Church | Armenia, Syria, Egypt |
| Donatism | North Africa | |
| Arianism | parts of Eastern Roman Empire until 380 Gothic tribes |
Constantine began to employ Christian symbols such as the Chi Rho early in his reign but still encouraged traditional Roman religious practices including sun worship. In 330, Constantine established the city of Constantinople as the new capital of the Roman Empire. The city would gradually come to be seen as the intellectual and cultural center of the Christian world.
Over the course of the 4th century the Christian body became consumed by debates surrounding orthodoxy, i.e. which religious doctrines are the correct ones. In the early 4th century, a group in North Africa, later called Donatists, who believed in a very rigid interpretation of Christianity that excluded many who had abandoned the faith during the Diocletianic Persecution, created a crisis in the western empire.
A synod was held in Rome in 313, followed by another in Arles in 314. These synods ruled that the Donatist faith was heresy and, when the Donatists refused to recant, Constantine launched the first campaign of persecution by Christians against Christians, and began imperial involvement in Christian theology. However, during the reign of Emperor Julian the Apostate, the Donatists, who formed the majority party in the Roman province of Africa for 30 years, were given official approval.
File:Nicaea icon.jpg|thumb|Icon depicting Constantine and the bishops of the Council of Nicaea. The centrally placed and haloed Emperor holds the Creed of the First Council of Constantinople.