I believe that the concept of the trinitarian dogma came from Athanasius
Before he was 20 years old, he wrote two treatises: “Against Pagans” and “The Incarnation” in which he argued that Jesus is God: “con-substantial and co-eternal with the Father ... as is the whole, so also is the part.” The Catholic Encyclopedia states that his two treatises “were admittedly written about 318 before Arianism as a movement had begun to be felt.”
In these two papers, Athanasius promoted Jesus past all the competition. He maintained that Jesus was not merely a man (because of his miracles); nor a prophet no matter how highly inspired. He was not a man who became a god, as Roman emperors were deified by vote of the Senate; nor a god who became a man, as Mithras was said to have done; nor a demigod like Hercules, the hybrid offspring of a god and a human mother; nor one of many gods, like Zeus; nor even the only-begotten, subordinate Son of the only God. None of these was sufficient. Athanasius said that Jesus was the “Artificer” who created all things in heaven and earth, the “re-creator” of human beings, and that while he was operating his human body, he was also operating and sustaining the rest of the universe as the Supreme Being. [Athanasius; Christian Classics Ethereal Library]
Athanasius apparently started with his conclusion, “Jesus is God,” and backed in all the arguments he could muster in support of that conclusion. The use of this rhetorical method is called sophistry. He justified his thesis in terms of its effect on believers: “Apart from belief in the Incarnation, we [they] cannot ultimately believe in Christ as Redeemer.”
And he used this in his battles against all other forms of Christian belief then existant until they were all declared as “Heresys” in 380 by Emperor Theodosius.
…27Then He said to Thomas, "Reach here with your finger, and see My hands; and reach here your hand and put it into My side; and do not be unbelieving, but believing." 28Thomas answered and said to Him, "My Lord and my God!" 29Jesus said to him, "Because you have seen Me, have you believed? Blessed are they who did not see, and yet believed." - John 20
Emperor Constantine’s Nicene council is usually pointed to as the source for the doctrine of the Trinity, with Athansius as the one who INFLUENCED him. Yet the Trinity was present in the church LONG before Constantine.
The term Trinitas was popularized by Tertullian almost 100 years before the Nicene council in his debate against Praxeas.
However, he was not the first to use the term, a man Theophilus Bishop of Antioch in 160 was the first to use the term (that we have in writing), many years before in his epistle to Autolycus The 2nd,xv.
We can assume it was used prior to Theophilus and was held as a common church belief with the many quotes that are left to us in history by the early church pastors. Athenagoras representing the whole Churches belief wrote, that, “they hold the Father to be God, and the Son God, and the Holy Spirit, and declare their union and their distinction in order.”(A plea for the Christians.10.3)
The term was used to simply describe the three that simultaneously exist as the one God. A man named Praxeas promoted what is called Monarchianism, which held a strict form of monotheistic progression. That the Father became the Son, and the Son became the Spirit. This is what is called modalism in it’s simplest form, What is better termed Oneness today. Despite the accusation’s of the Church inventing and promoting the Trinity. We find the Church in Rome and elsewhere falling prey to numerous heresies that they tried to keep out.
So, the Trinity did not depend on any council as it was used by Tertullian and others long before a council was called on doctrinal teaching.