The First Council of Nicaea, the first ecumenical council recognized by Christian churches (other than the Council of Jerusalem recorded in the New Testament), was summoned by Emperor Constantine in A.D. 325 in order to address disunity in the Church stemming from Arianism. It was the first time Christians could be called together from across the Roman Empire as it had been illegal before then and incurred persecutions leading to death.
The sessions opened between 20 May and 19 June. According to tradition, 318 bishops attended. They held discussions daily, not being discouraged or intimidated by the emperor's presence. (Constantine himself wished primarily for unity; he did not appreciate the importance of Arius's heresy.)
The church historian Eusebius, an Arian, offered the creed of his church, which did not address the questions in dispute, as a basis for agreement. The bishop of Nicomaeda, also an Arian named Eusebius, put forward a defense of Arianism. Arius himself was also heard and his points discussed; however, the overwhelming majority declared themselves in favor of the orthodox faith. Using Eusebius's creed as a base, they wrote the Nicene Creed which specifically and in detail asserted the divinity of Jesus Christ: Used by Eusebius of Caesarea, homoiousios means "of a similar substance". This is in contrast to the Nicene affirmation that Jesus and God the Father are homoousios, "of the same substance."  As expanded by the First Council of Constantinople, this creed remains the unified statement of faith of the majority of Christians to this day.
The Council of Nicea also issued twenty canons concerning church order and discipline. Modern skeptics such as Dan Brown have asserted that it also declared the official canon of the Bible; however, the main body of the canon (including all four Gospels) was already decided on long before the Council, and a few books only were still being discussed afterwards.
The 20 Canons of the Council of Nicaea demonstrate absolutely no syncretistic compromise with pagan religion, as some have charged. There is no evidence in any readings of the texts of the documents of the Council of Nicaea to support the charge that the emperor Constantine dictated to the council of bishops any change in doctrinal teaching or that he decreed or mandated any revision in any definition of Christian terms to accommodate any pagan beliefs, Roman or Greek or Babylonian.
- Catholic Encyclopedia: First Council of Nicea
- "THE PROFESSION OF FAITH OF THE 318 FATHERS"
- Theopedia: Homoiousios The only difference in spelling between homoiousios and homoousios is the single Greek letter iota ( ι ). From this comes the expression "an iota of difference": There is not an iota of difference here. How can it make an iota of difference? I don't care an iota about this. However, the difference here is profound.
- Eusebius of Caesaria, History of the Church
- See The Canons of the Council of Nicaea (christian-history.org) The commentary by the author rejects and even condemns outright some of the canons as being Catholic, sinful, and opposed to the Bible, but most significantly he says nothing about any pagan religious beliefs being evident in any of them.