sarai...
The gentiles were first brought into the Church by Peter in Acts chapters 10-11, but they were considered by many to be converts to Judaism. It was not until Chapter 15 that Paul (the main apostle to the gentiles) went to Jerusalem and met with the elders of the Church there that it firmly decided that new converts to Christianity were in no way expected to follow Judaic laws or traditions and were in no sense "Jews", although they shared in the blessings of the Church with full equality.
I think the "church" you refer to as beginning in the 4th Century at Nicea probably was the Catholic church, which is not the same thing. Catholics are like Methodists, Baptists, Episcopalians, etc., in that they are just different sects within professing christianity. The Bible has no such teaching, seeing all Christians as members of the Body of Christ, the Church.
mlsoft