From the non trinitarian perspective.....
"How Did the Trinity Doctrine Develop?
The Council of Nicaea did assert that Christ was of the same substance as God, which laid the groundwork for later Trinitarian theology. But it did not establish the Trinity, for at that council there was no mention of the holy spirit as the third person of a triune Godhead.
Constantine’s Role at Nicaea
FOR many years, there had been much opposition on Biblical grounds to the developing idea that Jesus was God. To try to solve the dispute, Roman emperor Constantine summoned all bishops to Nicaea. About 300, a fraction of the total, actually attended.
Constantine was not a Christian. Supposedly, he converted later in life, but he was not baptized until he lay dying. Regarding him, Henry Chadwick says in
The Early Church: “Constantine, like his father, worshipped the Unconquered Sun; . . . his conversion should not be interpreted as an inward experience of grace . . . It was a military matter. His comprehension of Christian doctrine was never very clear, but he was sure that victory in battle lay in the gift of the God of the Christians.”
What role did this unbaptized emperor play at the Council of Nicaea? The
Encyclopædia Britannica relates: “Constantine himself presided, actively guiding the discussions, and personally proposed . . . the crucial formula expressing the relation of Christ to God in the creed issued by the council, ‘of one substance with the Father’ . . . Overawed by the emperor, the bishops, with two exceptions only, signed the creed, many of them much against their inclination.”
Hence, Constantine’s role was crucial. After two months of furious religious debate, this pagan politician intervened and decided in favor of those who said that Jesus was God. But why? Certainly not because of any Biblical conviction. “Constantine had basically no understanding whatsoever of the questions that were being asked in Greek theology,” says
A Short History of Christian Doctrine. What he did understand was that religious division was a threat to his empire, and he wanted to solidify his domain.
None of the bishops at Nicaea promoted a Trinity, however. They decided only the nature of Jesus but not the role of the holy spirit. If a Trinity had been a clear Bible truth, should they not have proposed it at that time?
Further Development
AFTER Nicaea, debates on the subject continued for decades. Those who believed that Jesus was not equal to God even came back into favor for a time. But later Emperor Theodosius decided against them. He established the creed of the Council of Nicaea as the standard for his realm and convened the Council of Constantinople in 381 C.E. to clarify the formula.
That council agreed to place the holy spirit on the same level as God and Christ. For the first time, Christendom’s Trinity began to come into focus.
Yet, even after the Council of Constantinople, the Trinity did not become a widely accepted creed. Many opposed it and thus brought on themselves violent persecution. It was only in later centuries that the Trinity was formulated into set creeds.
The Encyclopedia Americana notes: “The full development of Trinitarianism took place in the West, in the Scholasticism of the Middle Ages, when an explanation was undertaken in terms of philosophy and psychology.”"
How Did the Trinity Doctrine Develop? — Watchtower ONLINE LIBRARY