The Council of Nicea was organised by Constantine the Great to end the Gnostic Schism that challenged the general teaching of the trinity by claiming that because Jesus was a man he was therefore less holy than God or the Holy Spirit. The first meetings defined the Niscene Creed which was basically a set of rules for what Christians believed in and then the Bible, which had grown to several dozen gospels, was culled of anything that disagreed with the Creed.
The Council was attended by all the Bishops of the Eastern Church and half the Bishops of the Western Church together with three legates from the Pope in Rome; even the Celtic Church of Ireland sent Bishops.
The decline of the Western Church was already in evidence for many of those attending were more administrators then theologians and the discussion was dominated by about six senior Eastern Bishops.
Although keeping two bodyguards for personal protection, Constantine did everything in his power to try and keep it a wholly clerical discussion, even going so far as to send half the city garrison away as a sign he didn't want the bishops to feel compelled to any particular view of his- other than that the Schism be ended one way or the other.
The Western Church was unsatisfied with the outcome and added the Apocrypha to its Bibles. Until the final break between East and West, the Pope maintained the fiction that these were not officially part of the Bible. Because this was a Papal decision the Lutherans and Calvinists ditched the Apocrypha as one of their first acts.