OK, well the history of unitarianism goes back to the beginning. The Bible was compiled by trinitarians but written by Jews, who are unitarians and reject trinitarianism to this day.
Hebrews use a different term, monotheism, but the difference is subtle. Although there is a church by the name, it no longer even considers itself Christian. So, the term 'unitarianism' began as part of the Protestant Reformation. It holds that God consists of 1 being.
Normally, defenders of trinitarianism reveal their true passion is the 'man is god' thesis. Focusing on the 3rd 'person', I hold there is no such person. That God, who is the Father, in his unitarian nature, is Spirit. His Spirit is Holy.
Now, as to the Son of God, which a reversal of terms has no legitimacy, his unique nature is indispensable, being Man's only mediator between us and God. Consider how the resurrected Jesus refers to his God in John 20:17 and Revelations 3 and the fact that the trinity is not in the Bible. There is only one God and that is Jesus' God. More to the point, there is no verse in the Bible that says something to the effect that the only way to be saved is to believe in the trinity.
I hold trinitarianism, a 4th century construct, violates the 1C & 2C.