State church of the Roman Empire

Nicene Christianity became the state church of the Roman Empire with the Edict of Thessalonica in 380 CE, when Emperor Theodosius I made it the Empire's sole authorized religion. The Eastern Orthodox Church, Oriental Orthodoxy, and the Catholic Church each claim to be the historical continuation of this church in its original form, but do not identify with it in the caesaropapist form that it took later. Unlike Constantine I, who with the Edict of Milan of 313 CE had established tolerance for Christianity without placing it above other religions and whose involvement in matters of the Christian faith extended to convoking councils of bishops who were to determine doctrine and to presiding at their meetings, but not to determining doctrine himself, Theodosius established a single Christian doctrine (specified as that professed by Pope Damasus I of Rome and Pope Peter II of Alexandria) as the Empire's official religion.

Earlier in the 4th century, following the Diocletianic Persecution of 303-313 and the Donatist controversy that arose in consequence, Constantine had convened councils of Christian bishops to define the orthodoxy, or "correct teaching", of the Christian faith, expanding on earlier Christian councils. A series of ecumenical councils met during the 4th and 5th centuries, but Christianity continued to suffer rifts and schisms surrounding the issues of Arianism, Nestorianism, and Miaphysitism. In the 5th century the Western 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 Eastern Roman Emperor Justinian I recovered Italy and other sections of the western Mediterranean shore. The Eastern Roman 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 Muslim conquests of the 7th century would begin a process of converting most of the then-Christian world in West Asia and North Africa to Islam, severely restricting the reach both of the Byzantine Empire and of its church. Missionary activity directed from Constantinople, the Byzantine capital, did not lead to a lasting expansion of the formal power of the Empire's state church, since areas outside the empire's political and military control set up their own distinct state churches, as in the case of in 919.

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