One of the debates surrounding the historical study of the New Testament is over the New Testament canon, and whether it should be embraced enthusiastically, embraced critically, or rejected.
This is because historians are supposed to be always looking for root causes and trying to examine the foundations of authority. But the NT canon–the 27 books we accept and no more–is an assumption, not something that can be proven. In the end, you either accept it or you don’t. And in the end, you either reject the other gospels and letters that different factions of the early church produced, or you accept them as equal to the documents in the canon. You will inevitably make a decision that is more about faith than it is about fact.
Michael Bird has a great post describing some of this debate, and the reasons for canonization in the early church. Here’s an excerpt: you should read the whole thing.
The canonisation of the New Testament was born of necessity and did not so much narrow the faith as much as prevent it from becoming so broad as to be nebulous. The formation of the canon was a historical process of theological judgments, one that was inevitable as it was essential. The question raised by persecution (which books do I hide from authorities) and deviation (which books should we not read at church) had to be settled as a pragmatic necessity, whether that was locally (e.g. Bishop Serapion in Antioch in relation to the Gospel of Peter ca. 180 AD), in a large diocese (Athanasius in his 39th Festal Letter of 367 AD), or regionally (councils of Hippo and Carthage ca. 393-419 AD). The New Testament’s canonisation represents the coalescing of a creedal and textual culture, however diverse it was across the east and west, that settled on the books of the New Testament as the normative attestation of the church’s faith. The formation and promulgation of the New Testament canon was not an arbitrary arrangement nor an exercise in pernicious politics. It constituted an effort to retrieve the christological foundations and apostolic testimony on which the church was based and to set limits on the extent of the diversity which it already knew.