BITINIA. – Western region of northern Anatolia, bordered to the north by the Black Sea, to the east by Paphlagonia, to the west by the Propontis (Sea of Marmara) and the Bosphorus, and to the south by Galatia and the province of Asia. Its most important centres were Chalcedon on the Bosphorus, Nicomedia (today İzmit) to its south, and further south Nicaea and Prusa (today Bursa), the ancient residence of the Bithynian kings. Occupied by the Romans in 74 BC after the death of the last king Nicomedes III, who bequeathed it by testament, it became one of the six Roman provinces of Anatolia, initially administered by the Senate and later by the emperor. In 65 or 64 BC, after the defeat of Mithridates, the western part of the Pontic Kingdom with the Paphlagonian coast was annexed to it, and the province was henceforth called Bithynia et Pontus. Yet the ethnic and religious differences between the two regions, Bithynia and Pontus, persisted: thus we find separate high priests and presidents of the people, namely the Bithynarch and the Pontarch.
Bithynia is one of the regions of Asia Minor where Christianity made the swiftest progress. It is mentioned in 1 Pt 1:1 among the recipients of the epistle, which suggests that it may have been evangelised by the Apostle Peter himself. Tradition holds that the first bishop was Prochorus, one of the seven first deacons. In the famous letter of Pliny the Younger to Trajan (Ep. 10 ad Trai.), we read that the Christian religion was widely diffused not only in the great centres but also in villages and the countryside, and that the persecutory laws against Christians exposed a great multitude of people of every age and condition to death (‘multi omnis aetatis, omnis conditionis’). On that occasion many shed their blood for the faith. Other martyrs suffered during the persecution of Decius, among them Tryphon, to whom a title church in Rome was dedicated. But the greatest number of martyrs occurred under Diocletian (v.), who in 285 had established his residence at Nicomedia and, at the instigation of his son-in-law Galerius, became the fiercest persecutor of Christians with the edict of 303. The great church of Nicomedia and other sacred buildings were destroyed; many were the martyrs. The rhetorician Lactantius (v.), whom Diocletian had summoned to Nicomedia to hold the chair of eloquence, was drawn to the Christian faith by the example of these martyrs. The persecution continued under Galerius and Maximinus Daia until the Edict of Milan (313) restored peace to the Church of Bithynia. Soon after, the Arian controversies flared up, and Bithynia was drawn into them, chiefly through the influence of Eusebius, bishop of Nicomedia. The First Ecumenical Council was held at Nicaea (v.) in Bithynia (325) to suppress the Arian heresy. Two other Ecumenical Councils were held in Bithynia: the Fourth at Chalcedon (v.) against Monophysitism (451), and the Seventh Ecumenical Council at Nicaea (787). Monastic life also flourished widely in Bithynia, and many monasteries were scattered throughout the region, particularly on Mount Olympus of Bithynia. The Persian invasion and the Turkish conquest nearly obliterated all traces of so glorious a past.