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, which he specified as that professed by Pope Damasus I of Rome and Pope Peter II of Alexandria, as the state's official religion.Earlier in the 4th century, following the Diocletianic Persecution and the Donatist controversy that arose following it, Constantine convened councils of Christian bishops to define an orthodox, or correct, Christian faith, expanding on earlier Christian councils. Numerous councils were held 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, with Rome being sacked in 410 and 455, and Romulus Augustus, the last nominal Western Emperor, being forced by Odoacer 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 Justinian I recovered Italy and other sections of the western Mediterranean shore. The empire soon lost most of these gains, but held Rome, as part of the Exarchate of Ravenna, until 751, a period known as the Byzantine Papacy. The Muslim conquests of the 7th century would begin a process of converting most of the Christian world in West Asia and North Africa to Islam, severely weakening both the Byzantine Empire and its church. Missionary activity directed from Constantinople did not lead to a lasting expansion of the 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 Bulgaria in 919.Justin I, who became emperor in 518, established the bishops of Rome, Constantinople, Alexandria, Antioch, and Jerusalem as the leadership of the Imperial church, referred to as the Pentarchy. By his time, the churches that now form Oriental Orthodoxy had already seceded from the state church, while in the west Christianity was mostly subject to the laws and customs of nations that owed no allegiance to the emperor. While eastern-born popes who were 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. Pope Gregory III (731-741) was the last to ask the Byzantine ruler to ratify his election. By then, the Empire's state church as originally conceived had ceased to exist. In the East, only the largest fragment of the Christian church was under the emperor's control, and with the crowning of Charlemagne on 25 December 800 AD as Imperator Romanorum by the latter's ally, Pope Leo III, the de facto political split between east and west became irrevocable. Spiritually, the Chalcedonian Church, as a communion broader than the imperial state church, continued to persist as a unified entity, at least in theory, until the Great Schism and its formal division with the mutual excommunication in 1054 of Rome and Constantinople. Where the emperor's power remained, the state church developed in a caesaropapist form, although as the Byzantine Empire lost most of its territory to Islam, increasingly the members of the church lived outside the Byzantine state. It was finally extinguished with the Fall of Constantinople in 1453.Western missionary activities created a communion of churches that extended beyond the empire, a communion predating the establishment of the state church. 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 Eastern Roman Empire, and among 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 state church came, by the time of the demise of the empire in 1453, to merge psychologically with it to the extent that its bishops had difficulty in thinking of Christianity without an emperor.Modern authors refer to this state church in a variety of ways: as the catholic church, the orthodox church, the imperial church, the imperial Roman church, or the Byzantine church, although some of these terms are also used for wider communions extending outside the Roman Empire. Its legacy carries on, directly or indirectly, in today's Catholic Church and Eastern Orthodox Church, as well as in others, such as the Anglican Communion.