I misspoke, being apostolic is valid. The Early Church answered how to know what books belong in the Canon. That those books which are apostolic belong in the canon of scripture. If a book had been handed down by the apostles as scripture (like the books of the Old Testament) of if it was written by one of the apostles or their associates (like the books of the New Testament), it belonged in the Bible. Apostolicity was thus the test for canonicity.
Protestant early Church historian J. N. D. Kelly writes:
“Unless a book could be shown to come from the pen of an apostle, or at least to have the authority of an apostle behind it, it was peremptorily rejected, however edifying or popular with the faithful it might be” (Early Christian Doctrines, 60). But how could one know which books were apostolic? Certainly not by a book’s claim to be apostolic, since there were many false gospels and epistles circulating under the names of apostles. Neitherdid the Holy Spirit promise a revelation to each individual Christian of what books belonged in the Bible.
But how was the test for apostolicity carried out in the early Church? Is I said in a earlier post Basically, there were two tests, both of them involving tradition. First, those books were reckoned as apostolic which agreed with the teachings the apostles handed on to the Church. Gnostic scriptures and other writings which did not agree with the apostolic tradition were rejected out of hand. This is something Evangelical scholars admit.
Protestant scripture scholar F. F. Bruce writes that,“[The early Fathers] had recourse to the criterion of orthodoxy…. This appeal to the testimony of the churches of apostolic foundation was developed especially by Irenaeus…. When previously unknown Gospels or Acts began to circulate… the most important question to ask about any one of them was: What does it teach about the person and work of Christ? Does it maintain the apostolic witness to him…?” (The Canon of Scripture,260).
Second, those books were regarded as apostolic which were preached in the various churches as being from the pen of an apostle or the associate of an apostle — not just its doctrines, but the book itself. If a given work was not regarded as apostolic and was not preached as such in the churches, then it was rejected. This was also an appeal to tradition because it looked to the tradition of the churches as a guide for apostolicity. If the tradition of the Churches did not recognize a book as apostolic, it was not canonized.
The fact that this was also used by the early Church to establish apostolicity is also something admitted by Protestant scholars. F. F. Bruce writes:“It is remarkable, when one comes to think of it, that the four canonical Gospsels are anonymous, whereas the ‘Gospels’ which proliferated in the late second century and afterwards claim to have been written by apostles and other eyewitnesses. Catholic churchmen found it necessary, therefore, to defend the apostolic authenticity of the Gospels…. The apostolic authorship of Matthew and John as well established in tradition. But what of Mark and Luke? Their authorship was also well established in tradition” (The Canon of Scripture, 257).
But of course not all of the Churches agreed. Some Protestant apologists are fond of pointing out that the Muratorian fragment, an early canon list dating from the A.D. 170s, includes most of the New Testament. But they fail to point out that the Muratorian fragment also omitted certain works from its canon. It did not include Hebrews, 1 and 2 Peter, and 3 John. Furthermore, it included works that the Protestant apologists would not regard as canonical: the Apocalypse of Peter and the Wisdom of Solomon. So there was obvious disagreement on the extent of the canon.
Eventually, the New Testament canon was settled at the Council of Rome in the year 382 under Pope Damasus I. Up to this point, its specific books were not firmly settled.
