Trinity

=== First Council of Nicaea (325) ===
{{Main|First Council of Nicaea}}
[[File:Glória de São Nicolau - António Manuel da Fonseca (Igreja de São Nicolau, Lisboa), cropped.png|left|thumb|''The Glory of Saint Nicholas'', by [[António Manuel da Fonseca]]; [[Saint Nicholas|Nicholas of Myra]], a participant in the First Council of Nicaea, achieves the [[beatific vision]] in the shape of the Holy Trinity.]]
In the fourth century, [[Arianism]], as traditionally understood,{{efn |name=Arius}} taught that the Father existed prior to the Son who was not, by nature, God but rather a changeable creature who was granted the dignity of becoming "Son of God".{{sfn|Cross|Livingstone|2005|p=100}} In 325, the [[First Council of Nicaea]] adopted the Nicene Creed which described Christ as "God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father", and the "Holy Ghost" as the one by which "[[Incarnation (Christianity)|was incarnate]]&nbsp;... of the [[Virgin birth of Jesus|Virgin Mary]]".<ref>{{cite web|url=http://www.ccel.org/ccel/schaff/creeds1.iv.iii.html|title=Creeds of Christendom, with a History and Critical notes |volume=I. The History of Creeds |website=Christian Classics Ethereal Library}}</ref><ref>{{cite web|url=http://www.creeds.net/ancient/niceneg.htm|title=The Nicaeno-Constantinopolitan Creed|first=Michael|last=Anderson |website=creeds.net}}</ref> ("the [[Logos#Christianity|Word]] was made flesh and dwelled among us"). About the Father and the Son, the creed used the term {{lang|grc-Latn|homoousios}} (of one substance) to define the relationship between the Father and the Son. After more than fifty years of debate, {{lang|grc-Latn|homoousios}} was recognised as the hallmark of orthodoxy, and was further developed into the formula of "three persons, one being".

The Confession of the First Council of Nicaea, the Nicene Creed, said little about the Holy Spirit.<ref name="BEoWR"/> At the First Council of Nicea (325) all attention was focused on the relationship between the Father and the Son, without making any similar statement about the Holy Spirit. In the words of the creed:

{{blockquote|We believe in one God, the Father Almighty, Maker of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God,] Light of Light, very God of very God, begotten, not made, being of one substance with the Father; ... And [we believe] in the Holy Ghost. ...}}