Hmmm .... I wonder what they used as the primary source in the early decades/centuries before any of the New Testament was even written.
That would be the Old Testament Scriptures and, of course in the Diaspora, that would be the Septuigent. The Reformers, however, was fascinated with the new Hebrew learning and so deleted much of the Septuigent from their canon. The Reformers were so eager to get rid of the Vulgate and so caught up in the novelties of the new Scriptural scholarship, that they bought the bill of goods that Erasmus sold them: a Greek Testament based on inferior manuscripts. By and large the Vulgate in use in the 16th Century was a better translation than the vernacular translations based on the best Greek mauscripts available.
I guess in part it depends on what you think about the nature of God. If you believe in a God who took on human nature in the form of Christ, and spoke through Christ to all men, then would you not look to those Apostles and teachers whom Christ had sent forth?
It goes without saying that these Apostles who had walked beside Christ were all alive when they wrote the letters of the NT, and so would have been available as living sources to Christ’s words and teachings. Their letters (which were later gathered as the NT) would then have been copied and available after they each were killed for their faith.
It must have been a humbling and profoundly moving experience to meet and talk to someone who had lived and walked with Christ. It must also have been comforting to know that others who had witnessed and could corroborate their statements were still alive at that time.