The Council of Nicaea (325 AD) settled one central question — whether the Son of God is of the same divine substance as the Father — by affirming the homoousios formula against Arius, who taught that the Son was a created being, the highest creature but not co-eternal God. What it did not settle was the decades-long controversy that followed: a majority of Eastern bishops resisted the term homoousios as philosophically suspect, and "Arian" Christianity dominated the imperial church for much of the mid-fourth century until Theodosius I enforced Nicene orthodoxy in 381. The council also addressed the date of Easter and the Meletian schism, though theology eclipsed these decisions in historical memory.
The conciliar text, with its explicit condemnation of those who say the Son is "of a different substance" or "created or changeable" — the first time an ecumenical council attached anathemas to doctrinal positions.
Defense of Nicaea written during the long post-Nicene controversy, explaining why homoousios was necessary and not merely a philosophical imposition — the most important patristic apology for Nicene Trinitarianism.
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