Hi Robert,
I agree that in general they did not disagree on the 27 books we have today (even though they did disagree on Revelation, Hebrew, James and Jude) so whoever taught you that they didn’t disagree AT ALL on what was Scripture did not teach you well.
But I think I get the gist of your point. Christianity KNEW the core epistles were Scripture and there were some epistles that didn’t make it into Scripture that were disputed.
I am not a historian, when it comes to reading and understanding the truth of Scripture, and I purposely avoid being one, because I have seen it wreck the pure understanding of the pure Word of God, by being tainted with other things outside of Scripture. It is not wrong in itself, but we can certainly see here, how it can lead to justification for rejecting Scripture as the authority of God's way, truth, and life.
The fact is, that all the books of the Scripture were already written and known by the end of the 1st century. There was no disputing the writings of the prophets and the apostles as being Scripture.
Even as Scripture prophesied, there would arise others as false christs, apostles, and teachers, even during the times of the apostles, who would put forth their own doctrines and prophecies for that of Christ.
Those concurrent and later writings were judged to be not Scripture itself, by being compared with the Scriptures that were already known: Sola Scriptura.
This understanding of the preservation of all Scriptures from God does not take scholarly study, but rather faith in the true God to do so, along with reasonable sense of how it would be done.
The end result of the Bible, as we have it today, proves it to have occured properly according to God's will. If there were errors and contradictions of any sort therein, then undoubtedly God's Scriptures did not make it to us today, as originally written by His prophets and apostles.
The main thing is that the good leaders of Nicea were lied about later on by false christian religion founders, whose sole purpose was to undermine Scriptural authority of God, in order to insert their own favored 'Sacred' writings and traditions as equal with Scripture of truth. And in the end, their 'Sacred' junk is more important to them than the Holy Scriptures of God.
These outside sources are all of the serpent's subtle work, and men are more than glad to go along with it out of the same pride, that Lucifer fell by: librarian keepers, as the scribes of old, were not content to be the guardians of God's written words, but wanted to elevate themselves alongside and then above what was written by the true prophets and apostles of Christ:
And these things, brethren, I have in a figure transferred to myself and to Apollos for your sakes; that ye might learn in us not to think of men above that which is written, that no one of you be puffed up for one against another. (1 Cor 4)
Thinking of men
above that which is written: Scripture, is the blindness of false religions made in the name of the Lord God of Israel and Jesus Christ, whether they call themselves Jews or Christians.