The first ecumenical council of the Christian Church, convened in AD 325 at Nicea (modern İznik, Turkey) under the emperor Constantine. Approximately 318 bishops attended. Its chief business was Arianism — the teaching of the Alexandrian presbyter Arius that the Son was a created being, "there was a time when he was not." The council decisively rejected Arius and produced the first draft of what would become the Nicene Creed, including the key term homoousios ("of the same substance") to describe the Son's relation to the Father.
Nicea was not a corruption of early simple faith into Greek philosophy — it was a defense of apostolic faith against a brand-new philosophical corruption. Arius taught a subordinationism that made Jesus a demigod; the council insisted Jesus is God in the full sense, with the same divine substance as the Father. The hero of the council (though a mere deacon then) was Athanasius of Alexandria, who later spent decades in exile defending Nicene orthodoxy against imperial pressure to compromise. The battle cry Athanasius contra mundum — "Athanasius against the world" — captures the cost of holding the line. Arianism keeps coming back: Jehovah's Witnesses and Mormons both teach variants of it today. Every time a religious teacher says "Jesus is a god, but not THE God," they're reading from Arius's old manuscript. Nicea settled the question for catholic Christianity seventeen centuries ago.