One reason that the process of canonization started was that there were early Christians who started making claims that most church leaders didn't agree with. During the second century, there was a religious leader named Marcion who thought that the only books that should be included in the canon were some of Paul's letters and part of the Gospel of Luke. The leaders of what would become the Catholic Church, on the other hand, thought that Marcion had it wrong, and they branded him a heretic. There were several other interpretations of Christianity which the leaders of the church judged to be wrong-headed. For example, there were "gnostics," people who thought that our bodies are evil and that our embodied existence is the result of some divine mistake. The Christian gnostics thought that the merciful God of the NT was not the creator of this world.
For various reasons such as these, the church leaders began to put together a list of works which should be included. For example, they decided to include four gospels rather than one. Raymond Brown points out three reasons for the preservation of particular early Christian writings:
1) An important criterion for inclusion was apostolic authorship; works could be included if they were believed to have been written by an apostle (e.g., Matthew or Paul), or at least the disciple of an apostle (e.g., Mark, who was thought to have been the interpreter of Peter).
2) Another factor which had a role in the canonical preservation of certain texts was the importance of these texts for particular Christian communities. Those communities which had influential churches, such as Rome and Antioch, were more successful in preserving those texts which they considered important.
3) Specific works were also subjected to the "rule of faith." Put dfferently, a work was likely to be excluded if it conveyed ideas that were considered contary to the teachings of the church. For example, the Gospel of Thomas never made it into the canon. One reason for its exclusion may have been its gnostic tendencies. Likewise, a document called the Infancy Gospel of Thomas was excluded, probably because it depicted Jesus as a wonder-working, mischevious, and even murderous child.
Hebrews almost didn't make it in; the leaders of the Latin speaking western church wanted it out, but the leaders of the Greek speaking eastern church wanted to keep it. Likewise, Revelation almost didn't make it. The eastern church wanted it out, but the western church insisted on its being included.
The process of canonization was fairly compete by the end of the 4th cen.
Some scholars have aruged that the formation of the canon was simply the result of a power struggle, but to formulate the issue in these terms is an oversimplification. Rather, as Christianity was becoming more and more diverse, and as it was intermingling to greater and greater extents with the numerous religions of the Roman Empire, various ideas began to emerge which the Church leaders thought were out of sync with the teachings of Jesus and the apostle Paul. If Christianity were to remain a distinct religion, its leaders had to indicate some guidelines.
Raymond Brown writes, "Although contributing to the preservation and importance of certain writings, these factors scarcely do full justice to what also seems to have involved a church intuition as to what was Spirit-guided" (p.12).
