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.
And their words were passed down through apostolic succession as well as their written words.