The NT is a product of apostolic preaching, tradition and theological insight post resurrection. There were Christians decades before any NT writings.
The NT is basically the tradition of the Church. There might of been some writings being circulated (speculated sources eg Q)but the Gospels and letters of Paul came later. Think about the priority God placed on the Church considering these communities of believers were preaching and believing long before the NT was written. The Bible is the inspired Word of God but if you think about the process that God chose and the fact that the Chuch played the role it did its clear that the tradition of the Church cannot be overlooked when trying to understand the NT. Do you believe the NT writings that the Evangelists wrote existed the first 20 or 30 years in the form that we have them today? Check out some of these books, they have been very helpful to me in gaining a better understanding of the NT and early Christianity. They are written by both Protestant and Catholics.
moses.creighton.edu/harmless/bibliographies_for_theology/Index.htm
P.R. Ackroyd & C.F. Evans, eds., Cambridge History of the Bible, 3 vol. (Cambridge: Cambridge University Press, 1963-1970) paperback. Dated, but valuable articles.
Paul M. Blowers, The Bible in Greek Christian Antiquity (Notre Dame: University of Notre Dame Press, 1997).
Harry Y. Gamble, Books and Readers in the Early Church: A History of Early Christian Texts (New Haven: Yale, 1995) paperback, $25. A superb and original study of how early books were made, collected, and used.
Christine Helmer and Christof Landmesser, eds., One Scripture or Many?: Canon from Biblical, Theological, and Philosophical Perspectives (New York: Oxford University Press, 2004) hardcover, $95.
Larry W. Hurtado, The Earliest Christian Artifacts: Manuscripts and Christian Origins (Grand Rapids, MI: Wm. B. Eerdmans, 2006) paperback, $20
James L. Kugel & Rowan A. Greer, Early Biblical Interpretation, Library of Early Christianity 3 (Philadelphia: Westminster, 1986).
Joseph T. Lienhard, The Bible, the Church, and Authority: the Canon of the Christian Bible in History and Theology (Collegeville, MN: Liturgical Press, 1995) paperback, $18.
Bruce Metzger, The Canon of the New Testament: Its Origin, Development, and Significance (New York: Clarendon / Oxford University Press, 1987) paperback, $20.
Bruce Metzger, The Early Versions of the New Testament: Their Origins, Transmission, and Limitations (Oxford: Clarendon Press, 1977) hardcover, $45.