Rebecca Lyman
The Council of Nicaea in 325 was a critical theological and institutional watershed between the local and often diverse theologies of one God as Trinity in the second- and third-century Christian communities and the universal or catholic credal statements of the ancient imperial church that developed over the course of the fourth century.
For the first time bishops from throughout the Roman Empire gathered, at the request and expense of the Emperor Constantine, to debate and declare their beliefs with an expectation of enforcement and unity – i.e. a council of broader representation and authority with a synodal creed.
While the origins of the theological conflict in Alexandria that prompted the gathering at Nicaea remain unclear, the decisions at the council and the extensive debates in the decades afterwards on the nature of God created the theological and exegetical foundations of Nicene trinitarian doctrine.
No longer imagined through a hierarchical divinity based on causality or separate spheres of activity, the one divine nature and operation of the Father, Son, and Holy Spirit was the eventual result of the extensive theological and exegetical work of the pro-Nicene theologians.
Recent studies have been especially attentive to the development of the theological methodology which underlay the defence and exposition of these foundational doctrines. The declaration and enforcement of Nicene orthodoxy by Emperor Theodosius in 381 marked the end of the formal debate, though dissent and persuasion continued.
The history of Nicene theology is therefore institutionally complex as well as theologically foundational.
Taken from St Andrews Encyclopaedia of Theology
The First Nicene Creed
Literal Translation
We believe in one God the Father Almighty, Maker of all things visible and invisible; and in one Lord Jesus Christ, the only begotten of the Father, that is, of the substance [ek tes ousias] of the Father, God of God, light of light, true God of true God, begotten not made, of the same substance with the Father [homoousion to patri], through whom all things were made both in heaven and on earth; who for us men and our salvation descended, was incarnate, and was made man, suffered and rose again the third day, ascended into heaven and cometh to judge the living and the dead. And in the Holy Ghost. Those who say: There was a time when He was not, and He was not before He was begotten; and that He was made out of nothing (ex ouk onton); or who maintain that He is of another hypostasis or another substance [than the Father], or that the Son of God is created, or mutable, or subject to change, [them] the Catholic Church anathematizes.