The Council of Nicaea made the official Church position (note, I do not say Catholic, as there was no such denominational split at the time, apart from some Eastern Churches in some schismatic disagreements. The point of the Council was to resolve such differences.) of which books were considered part of the Cannon, only in that after 325 years, these were the books that the majority of Christians' and their various Churches and leaders of the Churches throughout the known world were using. In a way, that was a type of grassroots democracy as to which books were used by early Christians.

The later decision by Luther on which books to remove from that Canon is quite interesting. For instance, Luther decided to abolish the 2 books of Maccabees which have various verses that support the Catholic notion of the Litany of Saints, i.e. prayers to the dead, etc. Supposedly, scholars claim that these two books were used by traditional Jews circa 2 millennia ago. Best throw out anything that does not conform to your worldview, eh?

Luther even wanted to go so far as to want to ban the Book of James. This is due to James 2:14, which states, "What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him?" Alas, this did not actually occur.

This particular verse did not sit well with the concept of Sola Scriptora, hence why Luther wanted to be rid of that particular book. Not entirely sure why / how the Book of James managed to survive.

Edit to quote James 2:24, "You see then that a man is justified by works, and not by faith only." and James 2:26, "For as the body without the spirit is dead, so faith without works is dead also."