It is widely believed, even among Christians and some of their scholars, that the Catholic Church established its official doctrine of the Trinity at its Council of Nicaea in 325 AD. But this is incorrect. The traditional doctrine of the Trinity as stated in English—that God is one essence existing as three co-equal and co-eternal Persons: the Father, the Son (Jesus Christ), and the Holy Spirit—was unknown then and for decades later. Although the Nicene Council did not provide minutes of its meetings, there is no evidence that it ever used the word “trinity,” which does not appear in the Bible, or discussed the nature of “the (Holy) Spirit.”
The main purpose of the Council of Nicaea was to resolve a debate within the Catholic Church which was threatening the peace of the Roman Empire. It was whether Jesus is fully God or essentially subordinate to God the Father, thus not equal to the Father in essence. It is quite clear in the writings of the church fathers of the second and third centuries, who were called “apologists,” that they subscribed to the belief that Jesus was essentially subordinate to the Father. Thus, they could call the Father “God Almighty,” yet refrain from doing the same regarding Jesus. Origen even called Jesus a “second god.”
The Council of Nicaea produced a “Nicene Creed.” It has been superseded in the history of Christianity only by “the Apostles Creed.” The primary thrust of the Nicene Creed is that Jesus is “very God of very God.” This terminology was borrowed from Greek philosophy. It means Jesus is equal to the Father in essence. And the Creed only says concerning the Holy Spirit, “We believe … in the Holy Spirit.”
The standard text on the history of this debate about the identity of Jesus and the church’s doctrine of the Trinity is R. P. C. Hanson’s magisterial book entitled The Search for the Christian Doctrine of God: The Arian Controversy, 318-381 (1988, 953 pp.).
The main reason people think the doctrine of the Trinity is in the Nicene Creed of AD 325 is that in many churches, including large church denominations such as the Anglican/Episcopal Church, the creed created at the Council of Constantinople in AD 381 is read in these churches during many of their Sunday morning worship services, and it sometimes is represented wrongly as being the Nicene Creed. In fact, this Creed of Constantinople is the Nicene Creed altered and expanded to include the doctrine of the Trinity even though it does not use the term “trinity” which is obviously because it is not in the Bible.