Claim: The New Testament canon as it currently exists is in a substantially reduced form from widely-accepted Christians writings from the first generation of the church, in particular neglecting gnostic writings. The Council of Nicaea established the canon because either the bishops whose views won out therein or Constantine did not prefer gnostic works.
Reality: The Council of Nicaea did not discuss the canon of the New Testament. The Athanasian and Arian sides were seemingly in agreement over the canon. Nicaea was convened principally to discuss matters of Christology, with the dating of Easter as a secondary issue.
The New Testament canon as it currently exists is largely in agreement with the set of works quoted by Ignatius of Antioch, Polycarp, and Irenaeus (all of whom died decades if not over full century before Constantine was born), along with the Muratorian fragment, written perhaps a full century before Constantine was born.
Ignatius quoted or alluded to most of the current New Testament canon (including many of Paul’s letters and perhaps all four canonical gospels), and, except Revelation, those books he did not were very short. He was martyred at the latest in the 140s, about 130 years before the birth of Constantine.
Polycarp quoted or alluded to most of the current New Testament canon as well, only neglecting John among the gospels. For what it is worth, he quotes two of John’s epistles. Again, most of the books he did not cite were very short in length, and there is only one known surviving work of his, an epistle to the Philippians. He was martyred in the middle of the second century, about 120 years before the birth of Constantine.
Unlike Ignatius and Polycarp, who contended with the proto-gnostics, Iraeneus had to contend with a more developed expression of gnosticism, along with Marcion and his followers, who promoted a very reduced form of the New Testament. Iraeneus wrote an idiosyncratic reasoning for why no fewer and no more gospels were to be considered canon than the now-accepted set of four, but given the quotations from earlier church fathers, it seems he was rationalizing a set of gospels that was already in wide use. He quoted or alluded to every book presently in the New Testament, with the exception of a few short, non-Pauline epistles. Note that he also considered 1 Clement and the Shepherd of Hermas to be Scripture. He died at an unknown date, under unknown circumstances, but likely sometime in the early third century, well before the birth of Constantine.
The Muratorian fragment is a name used to refer to the collation of two obscure manuscripts found in Italy containing a list of the books accepted at the time its original text was written as being in the New Testament. The manuscripts themselves are of medieval origin, but were copied from a text that refered to the Shepherd of Hermas as being a recent work, and attributing it to the brother of Pius, bishop of Rome AD 140-154. This suggests the work was originally written around AD 170, a century before the birth of Constantine. As the name suggests, the whole of the text which has survived to our day is in a fragmentary condition, but it mentions Luke and John being a third and fourth gospel, in addition to recognizing thirteen of Paul’s epistles, two epistles of John (which of them was included is unknown, but the Muratorian fragment quotes 1 John, suggesting either 2 or 3 John was excluded), the epistle of Jude, and Revelation. The only two books considered in the fragment not included in the canon are the Apocalypse of Peter and the Wisdom of Solomon. The former was described by the author as being rejected by some churches. The latter is included in the Old Testament by some groups. Hebrews, James, and the epistles of Peter were not discussed in the fragment. While the inclusion of Matthew and Mark as the first and second gospels is speculative, it is ludicrous to suggest the Gospel of John, which begins with describing Jesus as a fleshly being, Colossians, which describes the fullness of God dwelling in Jesus bodily, or the epistles of John, any two of which would have collectively described the protognostics as antichrist at least once, would coexist in the same canon as any gnostic gospel.
The only writings for which there seems to be some continuing disputes into the late second century were Philemon, Hebrews, some of the general epistles, and Revelation. However, besides these and the rest of the accepted New Testament, the closest to a somewhat accepted book in the New Testament canon was the Shepherd of Hermas, a decidedly non-gnostic work that was rejected due to its obvious post-apostolic origin and disconnection in concepts from the rest of Scripture. The overall trend, then, was for the canon of the New Testament to expand to its current form.
