One of the most fascinating topics in Early Christianity is the process by which Christians came to chose what went into the New Testament and what was excluded. This is a complex process that takes many centuries, and is characterised (like much of Early Christianity) by division, name-calling and power plays.
Initially there were just individual books and letters. Some of the things we now refer to as letters (such as Hebrews) were other things (a sermon, probably in this case). Other things were compilations of texts (2 Corinthians is likely to have been 2 letters originally).
Some of these documents began to be copied and circulated in groups. The letters of Paul being the earliest documented example of an anthology. But these collections were just ad-hoc collections. They didn’t have the status of a canon of scripture: a set of absolutely ‘right’ books. Some gospels (we’re not sure which) are mentioned as being revered at the turn of the C2 (95-105 ish) but not in canonical terms, only as valuable works.
The first person to suggest a canon was the heretic Marcion in the mid C2 (140ish), who took a sample of Paul’s letters, and added selected extracts from Luke’s gospel. The word ‘canon’ means ‘measuring stick’ and it was intended as a core of text from which doctrines or practice could be measured. But this approach didn’t catch on with mainstream Christian communities.
It is about the same period when we start to get writings that clearly favour the four gospels as more reliable. Iranaeus, for example, refers to this as the ‘Tetramorph’ (“4 forms”).
Origen, 50 years later, returns to the idea of a canon. His choice is recognizable as a NT, but has some differences, lacking James, 2 Peter, 2 and 3 John, and including the Shepherd of Hermas. The latter is a very widely revered book among early Christians that was in serious running for the NT for a long while, along with another text, the Didache. Both these texts featured in many of the canon lists of the next 100 years, where the books of Hebrews, James, 2 peter, 2 and 3 John and Revelation were often omitted because of dubious provenance.
In 325 the Church changed for good. The council of Nicea marks the end of the Early Christian period and the start of Christianity as a state religion (of the Roman Empire). This period is marked with numerous codifications on all kinds of levels: from doctrines, to church hierarchies, from membership lists to lists of canonical texts.
In 367, Anthanasius is the first to suggest exactly our 27 New Testament books. He posited a two-level scriptural authority, with the 27 books in his canon, and a further supplementary set of 6 books (including the Didache and The Shepherd of Hermas) in a kind of ‘required reading list’.
Athenasius’s 27 may be nothing more than a coincidence, since his Old Testament Canon isn’t that which was later adopted. Nontheless, in the following thirty years the canon lists seemed to iterate around this set and finally at the North African Synod of Hippo in 393 (some 360 years after Jesus, let’s not forget, and almost 300 years after the books were written), the New Testament Canon became an official part of Christian doctrine. And despite the efforts of Athanasius and the devotion of so many early Christians, books such as the Didache and The Shepherd of Hermas began their slide into obscurity.
As with all these things, there was opposition, so the decision had to be made again at both the Councils of Carthage in 397 and 418. After 420, the canon was pretty much stable, although in the western Catholic church, the debates rumbled and the canon wasn’t made part of the dogma of the church (i.e. the official infallible core beliefs that all Christians must hold) until 1546.
By and large these debates were minor, the only serious challenge came in the reformation. Luther saw even the composition of the New Testament as an option for his reforming hand, and removed Hebrews, James, Jude and Revelation from the canon (a set of four books that had been dubiously included originally and long been poorly thought of by theologians). This was a reform too far, however, and it never caught on outside certain branches of German reformed theology. Still, in some German language bibles today, these four are grouped separately after the rest of the New Testament.
Evangelical Christian theology is based on the New Testament being the absolute Word of God, of course, which poses a difficult historical problem for the Canon. It was such a messy and debatable process, and the arguments used by all sides are so obviously factional (and certainly not evangelical in any sense), that this has caused some difficulty. To resolve this Evangelical theologians have put forward a set of 4 ‘criteria’ for determining if a book is part of the canon, it should be of apostolic origin (either directly by an apostle, or by one of their disciples); it should have been universally accepted by the early church; it should be documented as being used in liturgy; and it should be consistent with other Christian writings.
The joke of this is, of course, that these were never the criteria used to determine the Canon. And (conveniently) Evangelicals aren’t campaigning to have any books removed or added from Athenasisus’s 27. So you have to see these criteria as post-hoc rationalizations. Certainly it can be argued very strongly that the pastoral epistles aren’t really consistent with much of the rest of the NT, Revelation likewise, but for other reasons. John’s gospel is highly inconsistent with the others. The Didache was often used liturgically and had (at one time) universal acceptance, which is more than can be said for James and Hebrews. And we now know that very little of the NT had anything to do with any of the apostles, save the authentic letters of Paul.
Still, for the sake of those who believe that all authentic teaching must only come from the canon, and further that their canon is therefore the right one, it is better not to know too much about the fickle and contingent process that gave rise to the New Testament. Canons are like sausages and laws: for sanity’s sake it is better not to dwell on how they are made.