But you already admitted Trinitarianism requires a systematic study of the whole of Scripture. That it's elements exist in the disparate parts of the cannon proves it did exist apart from the whole, therefore it predates the whole.
Furthermore, there is no reliable record showing the apostles letters were universally disseminated and approved for individual Churches as you allude to with your statement regarding attacks from within and the putting down of heresies.
This is a false argument you are presenting. The letters were circulated. From them truth was understood. Doctrines were believed. It was during the time of attacks from within that the councils delineated the doctrine from all of Scripture.
He DID??