Atheists & Agnostics

The establishment of Christian canon was a gradual process that took off in the 2nd century, and was contemporaneous to a similar movement within Judaism. By Constantine's time, Gnosticism and Marcionism had already been pushed to the fringes of Christianity. The 27-book version of the New Testament had already been around for more than century at that point. The Council of Nicea and the Council of Rome weren't about eradicating Gnosticism because by that point Gnostics were largely a non-issue. They were more concerned with Arianism, which was a major point of debate at the Council of Nicea.