NO, it is not. Where did the council of Nicea get the evidence of the Trinity?
Yes, it is.
What do you think Nicaea was about anyways? It defined the trinity as we understand it today and affirmed the divinity of Christ and that you have God the Father, God the Son and God the Holy spirit, three persons in one godhead.
Rejecting Nicaea = rejecting the trinity.
There are non-trinitarians who claim to be christians. We call them ‘Mormons’ and ‘Jehovahs witnesses’.