Understanding the teaching of the Apostles through circulating letters laid out the Triune Nature of God.
When the church was attacked from within, it was formally laid out to guard against encroaching heresy.
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.