Arianism by the 320s has become so widespread in the Christian church and spurred such disunity that Constantine, prompted by Hosius, convokes the Council of Nicaea in May 325, the first ecumenical council held by the church, meant to settle the relationship between the persons of the Trinity. (This is not Constantine's first attempt to reconcile orthodox and heretical factions in Christianity, but it is his first use of the imperial office to impose a settlement.)
A lengthy and heated debate ensues among the attendees, nearly all of whom come from the eastern Mediterranean region.
Athanasius may have accompanied Alexander to Nicaea, who, called as a theological expert, leads the council in defending the unity of Christ as both God and man, promoting his “homoousios” ("of one substance") doctrine to establish the full divinity and equality of Christ with the Father, as against the Arian position of “homoiousios” ("of like substance").
Eusebius of Caesarea, having become embroiled in the controversy raised by Arianism over the nature of the Trinity, seeks to reconcile the opposing parties.
Not naturally a spiritual leader or theologian, but as a very learned man and a famous author who enjoys the special favor of the emperor, he comes to the fore among the three hundred members of the council and is prominent in its transactions.
The confession that he proposes becomes the basis of the formula approved at Nicaea, which Eusebius, although disinclined to fully support the “homoousios” doctrine propounded by Alexander and Athanasius, eventually signs, largely in deference to Constantine.
The council issues a decision, formalized in the Nicene Creed, declaring that God the Father and God the Son, or Christ, are of one identical and eternal substance.
The Arian belief in a Christ created by and thus inferior to the Father is thus deemed heretical, and Arius himself is excommunicated and banished.
The Nicaean Creed simultaneously rejects Monarchianism (the belief that God the creator is supreme but shared his power with Christ, the logos or Word) and Sabellianism, or Modalism (the belief that the three persons of the Trinity are modes or aspects of the same God).
The council also makes disciplinary decisions concerning the status and jurisdiction of the clergy in the early church and establishes the date on which Easter is to be celebrated.
Hosius is influential in securing the inclusion in the Nicene Creed of the key word homoousios, to affirm that God the Son and God the Father are of the same substance.
The Council, which represents the first stage in the rigidification of Christianity, officially changes the date of Easter from Passover and forbids Jews from owning Christian slaves or converting pagans to Judaism.
In this year also, Constantine outlaws gladiatorial combat in the Roman Empire.
Those who were condemned to become gladiators for their crimes are to work from now on in the mines.