Christianity spread to Nicomedia during the middle of the 1st century, while the city became the oldest bishopric established in the region of Bithynia, in northwestern Asia Minor. According to the Christian tradition, the first bishop was Prochorus, one of the Seven Deacons. Moreover, the first written account about the Christian community of the city is found in an epistle of Eusebius of Caesarea in 170 AD.
Nicomedia became a significant important administrative center during the reign of Emperor Diocletian. The latter aimed at transforming the city into a new capital of the Roman Empire. In 303, during the reign of the same emperor, the Christians of Nicomedia witnessed persecution, while another wave of persecution against the Christians occurred in 324 under Licinius. The number of the local Christian victims has not been estimated but it is believed that they were thousands. Among the martyrs were the legionaries Dorotheus, Gorgonius, Panteleemon and George, as well as the local bishop Anthimus. In 337 bishop Eusebius of Nicomedia baptised Emperor Constantine the Great on his deathbed.
At 451, the local bishopric was promoted to a metropolitan see under the jurisdiction of the Ecumenical Patriarchate of Constantinople. The metropolis of Nicomedia was ranked 7th in the Notitiae Episcopatuum among the metropolises of the Patriarchate.
The last attested metropolitan of the 14th century was Maximos (1324–1327). After 1327 the metropolitan see remained vacant, most likely due to the prolonged period of the Ottoman siege of the city. Nicomedia was the last city of Bithynia that remained under Byzantine