Sources: Eusebius, Life of Constantine 3.4–24; Athanasius, On the Decrees of Nicaea; Socrates Scholasticus, Ecclesiastical History 1.5–9; the Nicene Creed itself.

The Council of Nicaea (325 AD) is the first ecumenical council of Christianity, convened by Emperor Constantine to resolve the Arian controversy that was threatening imperial unity. Arius of Alexandria (~256–336 AD) taught that the Son of God was the first and greatest creature — “there was a time when he was not” — subordinate to and ontologically different from the Father. Alexander of Alexandria and his deacon Athanasius countered that the Son is co-eternal and of the same substance (homoousios) as the Father. At Nicaea, approximately 300 bishops met; the Arian position was condemned and the Nicene Creed formulated: the Son is “of one substance with the Father” (homoousios). Key outcomes: (1) The Creed — the first authoritative theological formula binding all Christians; (2) Easter date — unified calculation method (following the equinox, not the Jewish Passover calendar); (3) Meletian schism — a North African disciplinary dispute resolved; (4) Canon 6 — recognized the authority of Alexandria, Rome, and Antioch as major sees. What Nicaea did NOT settle: the council did not resolve the canon (a persistent myth), and Arianism was far from dead — it dominated large portions of the church for decades afterward, with several emperors being Arian. The creed was revised and expanded at Constantinople (381 AD) into the form still recited today.