What was the Council of Nicaea?

The quarrel that started it

Around 318 a popular priest of Alexandria, Arius, began to teach that the Son of God was the first and greatest of all creatures, but a creature still, with a beginning. His slogan was blunt: there was when he was not. His bishop, Alexander, answered that the Son is eternal, God from God, and threw him out. Arius took his case to friendly bishops across the East and even set his ideas to popular verse, the Thalia.

Constantine had just become sole ruler of the empire, in 324, and wanted one church behind him. He first wrote to both men, calling their dispute a quarrel over trifles (Life of Constantine 2.64–72). When that failed, he summoned a council.

Who came, and where

The bishops met in the imperial palace at Nicaea, a lakeside city in northwest Asia Minor, from about May to July 325, and the state paid their travel. Eusebius of Caesarea, who was there, counted more than 250. Athanasius, then a young deacon at Alexander’s side, said about 300. Later tradition settled on 318, the number of Abraham’s fighting men in Genesis 14:14. Almost all came from the Greek-speaking East. The West sent a handful, among them Ossius of Córdoba, Constantine’s adviser, and two priests representing the elderly bishop of Rome, Sylvester.

Eusebius describes the opening day. The emperor walked in last, robed in purple and gold, and would not sit down until the bishops signaled that he should (Life of Constantine 3.10).

What it decided

• The creed. The council declared the Son “of one substance,” homoousios, with the Father, “begotten, not made,” and condemned anyone who said there was when he was not. All but two bishops signed. Arius and the two holdouts were exiled to Illyricum, in the Balkans.
• Easter. Every church would keep Easter on the same day, worked out by Christian reckoning instead of the Jewish calendar. The council did not fix the formula; the detailed tables came later.
• Twenty canons. Short rules of discipline: at least three bishops to ordain a new one, the authority of Alexandria over Egypt and Libya, like Rome’s in the West, and even a rule to pray standing, not kneeling, on Sundays.

The council closed with a banquet for Constantine’s twentieth year in power. Eusebius thought the bishops dining with the emperor, between ranks of guards with drawn swords, looked like a picture of the kingdom of Christ (Life of Constantine 3.15).

What came after

Nicaea did not end the argument. Within a few years Constantine let Arius return, and in 337 the emperor was baptized on his deathbed by Eusebius of Nicomedia, a bishop who had once backed Arius. Athanasius, bishop of Alexandria from 328, was driven into exile five times for defending the creed. The fight lasted until a second council, at Constantinople in 381, confirmed Nicaea and enlarged its creed. The Nicene Creed said in churches today is, by tradition, that enlarged text; the original of 325 ended simply “and in the Holy Spirit,” followed by its curses on Arius’s phrases. One thing Nicaea did not do was choose the books of the Bible.

Nicaea is now İznik, a small town in Bursa province, still ringed by its ancient walls. Another council met there in 787, over icons, which is why this one is called the First. In 2014 the remains of a basilica built within about a century of the council were found in the waters of Lake İznik. In November 2025, Pope Leo XIV and Ecumenical Patriarch Bartholomew I marked the council’s 1,700th anniversary together in Turkey.