We have seen that very early on the four gospels were circulating as a collection. Also, Paul's letters were circulated together, and Acts began to circulate along with the gospels. Along with what became the canonical New Testament, letters from later authors such as Clement of Rome and Ignatius were also passed around. There seems to have been no thought given to providing a closed collection of "New Testament writings."
The advent of Marcion of Sinope (c. 140 CE) proved to be a turning point. By his period, considerable confusion was arising among gentile Christians regarding various issues. Marcion was among those who attempted to synthesize a coherent understanding of the entire Bible. Going through appropriate channels, he met with the older people (c. 144), attempting to explain his understanding of the Bible. Marcion rejected the Old Testament entirely, for like many in his period he was anti-Jewish. He apparently viewed the God of the Jews as an inferior god to the loving Christian Deity and saw Jesus as expressly rejecting points of the Torah.
Marcion collected his writings into a volume, Antitheses, and advocated a limited collection of inspired writings. First, he read how Paul spoke of "my gospel" and concluded that this "gospel" was a written work which Paul had been circulating. Considering that Luke was Paul's associate, Luke's account must have been the closest one to "Paul's gospel." Marcion edited Luke to conform with what he believed was the truth. Together with Paul's letters to the churches and Philemon, these constituted the entire Bible for Marcion, who had not only placed the "New Testament" writings equal to those of the Old, in Marcion's mind the writings of Paul superceded the Old Testament entirely. Some commentators say that it was Marcion who coined the expressions "Old Testament" and "New Testament."
Marcion's teachings were rejected by the older people, but a group of followers (called the Marcionites) began to agree with his ideas. Other Christians reacted to the teachings of Marcion with harsh disagreement. In particular, his treatment of the nature of God (as two separate beings) and of the Bible were attacked by his many opponents. They attacked his tampering with Paul's letters, and some of his adversaries opposed his leanings toward Gnostic beliefs. Justin Martyr, Irenaeus, and Tertullian became strong opponents of Marcionism. Lists of "books in use" began to develop at least in various areas of the Empire.
An early list that has survived today in the form of a late (8th century) copy is called the Muratorian Canon after its discoverer, Count Muratori. The copy we possess is broken at the beginning but apparently identifies Mark as being the second of four "gospel" accounts. It continues by listing the gospels, Acts, the Pauline letters, and most of the rest of the New Testament. The Canon explicitly mentions Marcion's sect as the author of two forged Pauline letters and includes the Wisdom of Solomon among the listed books. The author indicates partial acceptance for the Revelation of Peter and rejection of Hermas' Shepherd, which was written "recently." Click here to read a translation of the Muratorian Canon.
By the beginning of the fourth century, numerous new writings (mostly gnostic) were being circulated. Opponents of gnosticism became progressively more opposed to adding anything at all to the writings of the apostles. Although a letter by Clement of Rome and the Shepherd (of Hermas) were still widely accepted as canonical, other writings (such as the "Letter of Barnabas") were in the process of being rejected from the books accepted by Christians. By the time of Eusebius (c. 325), the canon was fairly well fixed, as an examination of his History of the Church shows. The next extant canon list is that of Athanasius of Alexandria, who listed all of the books then (367 CE) in use. He lists the general letters before the letters of Paul, but otherwise, his list conforms to the usual New Testament. In 397, the Catholic Church approved the entire New Testament for use in the church (at the Third Synod of Carthage). [Actually, the NT had been approved in 393, but nothing survives from that Synod (of Hippo).] The list appears to lack one of the Pauline letters--unknown as to which--but a reiteration of the statement (in Carthage, 419) mentions all of them.
Although Revelation was in doubt as late as the eighth century, the essential components of the New Testament were established and the canon closed as early as 367 CE. The Church of that time never had a single list of criteria for inclusion in the canon, but over the years a set of criteria developed. Generally speaking, a writing must have been written by an early Christian (prior to the time of Clement), agree with the rest of the writings, and have found acceptance in the Church.
Click here for a discussion of New Testament manuscripts and texts.