First Council of Nicaea

In the summer of 325, the Roman emperor Constantine I gathered Christian bishops from across his realm at Nicaea, a city in Bithynia that is today İznik in Turkey. Counted as the first ecumenical council, it set out to settle a bitter quarrel over how Christ the Son related to God the Father. It produced the original Nicene Creed, a common rule for dating Easter and a set of church laws, and it set a precedent for later councils that defined doctrine.

The quarrel began in Alexandria, probably sometime between 318 and 322. Arius, a presbyter, taught that the Father alone was eternal and that the Son had been begotten by the Father, so had a beginning and was subordinate to him. His bishop, Alexander, held that the Son was eternally generated from the Father. A local council of Egyptian and Libyan bishops backed Alexander and excommunicated Arius, who then looked for support elsewhere in the Roman east and found it among bishops such as Eusebius of Nicomedia and Eusebius of Caesarea. A separate schism, led by Melitius of Lycopolis, was also dividing the Egyptian church.

Christianity had only recently gained legal protection, after the Diocletianic Persecution and the Edict of Milan of 313. In 324 Constantine defeated his eastern rival Licinius and became sole ruler of the Roman Empire. Learning of the Alexandrian dispute, he wrote urging both sides to reconcile and sent Bishop Hosius of Corduba as his envoy. When that failed, a council was planned for Ancyra, but Constantine moved it to Nicaea, which lay close to his residence at Nicomedia and was easier to reach. The imperial treasury paid the bishops' travel.

Ancient reports put attendance at between 250 and 300 bishops; the figure of 318 later became traditional. The surviving signature lists hold roughly 200 to 220 names. Nearly all came from the eastern provinces, with only a handful from the Latin west, and Pope Sylvester I was represented by two presbyters. The council probably opened in late May or June, and Hosius most likely presided. Constantine opened proceedings with a speech in Latin and joined in debate, though as a layman he did not vote. No full minutes survive, so the order of business is uncertain.

The bishops adopted a statement of faith, drawn from earlier creeds, which described the Son as "begotten, not made" and "of one substance" with the Father, the Greek term homoousios. It closed with condemnations of Arian formulas. All but two bishops signed. Arius and the two dissenting bishops, Theonas and Secundus, were excommunicated, and Constantine exiled them to Illyria. The council also ruled that Easter should be calculated independently of the Jewish calendar, following the method of Rome and Alexandria, and that it should be kept uniformly everywhere. It placed limits on Melitius and issued about twenty canons on church discipline, including the recognised authority of the bishops of Alexandria, Rome and Antioch. The bishops dispersed in July, after attending Constantine's twentieth-anniversary celebrations.

Nicaea did not end the controversy. Supporters of Arius soon regained influence at court, and leading Nicene bishops, among them Athanasius of Alexandria, were deposed in the following decade. Arius was about to be readmitted to communion when he died, and Constantine himself was baptised shortly before his death the following year by Eusebius of Nicomedia. Two later emperors, Constantius II and Valens, favoured Arian positions. The creed was revised by the First Council of Constantinople in 381. By then, the use of imperial power to enforce doctrinal decisions had become an enduring feature of the Christian empire.

Our 300 map shows the Roman Empire shortly before Constantine reunited it, and the 400 map shows it split into the Western Roman Empire and an eastern half that included Nicaea.