By the early fourth century, a presbyter of Alexandria named Arius was teaching that the Son of God, though the highest of all creatures, was created — that 'there was a time when he was not.' His own bishop, Alexander of Alexandria, opposed him, and the dispute spread until it threatened the peace of the whole Church. Newly in control of a unified empire, Constantine summoned bishops from across the Christian world to settle it.
The Question
The council was not asked to invent a doctrine but to say clearly what the Church had always confessed: is the Son truly God, or a creature? The answer mattered for salvation. If the Son is a creature, then it is a creature who saves, and the worship the Church offered him would be idolatry. The bishops confessed instead that the Son is homoousios — of one substance — with the Father: not made, but eternally begotten, 'true God of true God.'
The Outcome
The council produced the original form of the Nicene Creed, condemned the teaching of Arius, and fixed the calculation of Easter. Among the deacons assisting was a young Athanasius of Alexandria, who would spend the rest of his life defending the council's confession through repeated exile. Arianism did not vanish at Nicaea — it contended for much of the century that followed — but the word the Church needed had been spoken and written down.