Christianity in Turkey
Constantinople became established in the ecclesiastical hierarchy at the Council of Constantinople in 381. Second only to Rome, the legendary origins of the Patriarchate of Constantinople go back to St. Andrew, Metrophanes and Alexander of Constantinople. Constantinople's primacy over the Patriarchates of Alexandria and Antioch was reaffirmed at the Council of Chalcedon in 481, after which the papacy in Rome supported Constantinople in its dispute with Alexandria over monophysitism. Later, when Rome sought to assert its primacy over Byzantium, the Eastern Orthodox church developed the doctrine of pentarchy as a response.
History of Istanbul
Constantine’s conversion to Christianity a few years earlier, in 312, had set the Roman Empire onto Christianization, and in 381, during the reign of Theodosius I, the official state religion of the Roman Empire became Christianity, which turned Constantinople into a thriving religious center. After the end of his reign in 337, Constantine declared his three sons as joint heirs of the Roman Empire in a system of co-emperorship. Unfortunately, the sons couldn’t govern together peacefully and their war raged rivalry split the empire on the north-south line along the Balkan Peninsula. The territory was officially split in 395 when Theodosius I (ruled, 379-395) died leaving his son Arcadius emperor of the eastern half of the empire and his other son Honorius emperor of the western half.