Bithynia
Archimandrite Nikifor’s Biblical Encyclopedia
(Acts 16:7) — a region in Asia Minor, bordered on the east by Paphlagonia, on the north by the Black Sea, on the south by Phrygia and Galatia, on the west by the Propontis and the Thracian Bosphorus. Within it were the city of Olympus and the cities of Nicaea, Nicomedia, Heraclea, Chalcedon, and others. Christianity appeared here as early as apostolic times (cf. Acts 2:9; 1 Pet. 1:1). In church history there is a notable testimony to the purity and firmness of the faith of the Bithynian Christians at the end of the first and beginning of the second century.
In the days of Pliny, who was governor of that province, and of Trajan (98 to 117 A.D.), there were already very many Christians here, not only in the cities but also in the towns and villages, so that the pagan temples were almost empty. At Nicaea, the chief city of Bithynia, the First Council of Nicaea was convened in 325 A.D., and at Chalcedon the Fourth Ecumenical Council of Chalcedon in 451 A.D.