[Esaias]

The books that were canonised by the catholic churches in their ecumenical councils were recognized as being in two categories: a) the books, epistles, etc that everybody accepted as being authentic, and b) certain books which were disputed, that is, some held the books in this second category as authentic Scripture, others did not. I believe this second group was primarily concerned with certain "old testament" books (generally known as deuterocanonical, or apocryphal, books).

So, the New Testament books/epistles were canonized because they were what practically ALL Christians everywhere used, whether trinitarian, Arian, "Sabellian", Montanist, etc.

I always ask, "What books aren't in the New Testament that should be?" I have read numerous proposed "lost books of the New Testament", and quite honestly none of them "fit": they either teach things clearly contradictory to each other and the NT, or they just read like goofy juvenile nonsense, or they are clearly gnostic mystical speculative writings.