One of the earliest Christian councils, the Council of Elvira, probably held near or in the present city of Granada in 305 or 306, decrees that all priests and bishops, married or not, should abstain from sexual relations; it also forbids Christians from intermarriage and social interaction with Jews.

Constantine, having conquered Italy and secured his regime as Emperor in 312, has begun promoting Christianity.

It is in these early years of Constantine’s reign that he begins issuing laws conveying upon the church and its clergy fiscal and legal privileges and immunities from civic burdens.

As he writes in a letter of 313 to the proconsul of Africa, secular offices should not distract the Christian clergy from their religious duties “… for when they are free to render supreme service to the Divinity, it is evident that they confer great benefit upon the affairs of state.” In another such letter, directed to the Bishop of Carthage, Constantine mentions the Spanish bishop Hosius.

Consecrated bishop of Córdoba around 295, Hosius had attended the Council of Elvira and from 312 has acted as Constantine’s ecclesiastical adviser.

Since he may well have been with Constantine in Gaul before the campaign against Maxentius, Hosius is possibly instrumental in the conversion of the Emperor to Christianity.

The Edict of Toleration, by which Constantine and his co-emperor Licinius officially recognize Christianity in 313, authorizes the toleration of different religions in the Roman Empire yet gives supremacy to Christianity.

Hosius convokes another synod, one of Syrian bishops, at Antioch.

Both this synod and the one held simultaneously at Alexandria condemn Arius and his followers.

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.

Bishop Hosius, who had been the ecclesiastical advisor to the late Constantine I, presides in the winter of 342-343 at the Council of Serdica (modern Sofia), which city had reached its greatest height under Constantine.

The council, called by Constans to settle the conflict between the orthodox Bishop Athanasius of Alexandria and the Arian Paul of Constantinople, achieves little, being boycotted by the Eastern bishops because the Westerners and the Egyptians insist on the presence of Athanasius the great defender of Trinitarian Christianity and the principal opponent of Arianism.

The Arian controversy, the continuation of which has been aided by the conflicting politics of the empire after the death of Constantine, has emerged as three distinct types by the mid-fourth century.

Radical Arianism holds that the Son was "dissimilar" to the Father; homoeanism maintains that the Son was similar to the Father; and semi-Arianism veers off into orthodoxy in its position that the Son was similar yet distinct from the Father.

Constantius passes laws against paganism, and pagans as well as orthodox Nicaeans (Homoiousians) and extremist Arians (Anomoeans) are persecuted, for in 356-357 several edicts proscribe magic, divination, and sacrifices and order that the temples be closed.

The homoean party scores an initial success in 357, after which the semi-Arians join the orthodox ranks.

From 353 to 356 Bishop Hosius, who had been ecclesiastical advisor to Constantius’s father, the emperor Constantine, has strongly resisted the efforts of Constantius to have Athanasius condemned by the Western bishops and in a famous letter reproves the Arian emperor for intruding into ecclesiastical matters.

Summoned to Sirmium in 356 and detained at court for a year, after threats and physical violence, Hosius signs the Arian formula of Sirmium in 357.

Hosius retracts his signature, however, before he dies in Córdoba late in the year.