When Constantine became the first emperor to rule the entire Empire since Diocletian, he hoped to use Christianity to unify the Empire. One of the main barriers, however, was the theological dispute between the priest Arius and the deacon Athanasius (both from Alexandria) over the nature of Christ. In an attempt to settle this, Constantine called on the existing Christian tradition of synods: for a long time beforehand, Christian bishops in different regions of the Roman Empire had gathered in meetings called synods to discuss issues of theology or church order. So, in an attempt to create the most grand and significant synod ever, Constantine summoned bishops from all over the empire to meet at Nicaea.

Athanasius wrote that 318 bishops attended the Council; there's no reason to doubt that figure (we can discard the later tales of thousands of attendees). Most of them were from the eastern part of the Roman Empire, though some came from as far away as Gaul and Persia. Their main intention was to settle the Arian controversy, though some issues of church order such as the dating of Easter (the same system we use today) and methods for priests to move between dioceses (not "recklessly," nor without the bishop's permission) were also settled. Contrary to modern popular belief, though, there is no record that they ever discussed the canon of Scripture; except for a few epistles and the Book of Revelation, it was universally agreed on well before then.

Regarding the main order of business, Arianism was roundly condemned as a heresy. Athanasius argued that Jesus must be fully and completely God, or else He could not be worshiped and His death could not save the world. These arguments won over many, including the noted church historian Eusebius, and the Nicene Creed was adopted over only five dissenting votes: Jesus is "true God from true God, begotten [gennethenta] not made [poethenta], of one substance [homoousion] with the Father." The Creed, with slight expansions, is still recited in many churches today.

In retrospect, perhaps this Council could be called the origin of the medieval church. It officially ruled the patriarchs of Rome, Alexandria, and Antioch to have preeminence over all other bishops in their areas, and it started the custom of summoning ecumenical councils to give authoritative rulings. However, it was not viewed that way at the time. The Council only refers to itself as a "great synod," and few accepted its ruling - even Constantine's sons themselves held to Arianism, and Athanasius was exiled multiple times for defending Nicene orthodoxy. It was not until sixty years later, when Emperor Theodosius summoned another ecumenical council at Constantinople to again, more efficaciously, condemn Arianism, that the significance of the Council of Nicaea began to be seen.