I was asked to come speak to some students at Missouri State in response to a recent "Skeptics" convention they had there. I was asked to respond to some of the questions that were raised among the members of that campus ministry. One of the things I was asked about was the formation of the New Testament. I pulled together some notes from Introducing the New Testament by Mark Allan Powell. I will post part one of the development of the canon here and part two later.
Canon literally means "rule" or "standard", but it is used by religious groups to refer to a list of books that are officially accepted as scripture. In the early years, Christians simply gathered together writings that they found to be helpful and shared them with each other. Paul encouraged the churches to which he wrote letters to exchange those letters with each other, so that they could read what he had written to other congregations as well as to their own community (see Colossians 4.16). It seems that multiple copies of Mark's gospel were produced and distributed to different parts of the Roman Empire a few years after it was written (both Matthew and Luke appear to have had copies). The works that circulated were the writings produced by people who had known Paul or those original disciples of Jesus, or at least people who had known Paul or those original disciples. This chain of connection to Jesus and Paul would come to be known as the "apostolic tradition."
From the start, however, there were voices within Christianity that were in tension with that developing tradition. there were people arguing for versions of the Christian faith that Paul himself rejected (see Galatians 1.6-9). Some of these alternative voices probably produced writings as well (see 2 Thessalonians 2.2 for evidence) but their works do not appear to have been preserved or included in the NT. The NT, then, is not just a collection of early Christian writings; rather it is a selection of those writings. The NT contains those works that were considered to be most representative of what became mainstream and orthodox.