The New Testament canon is based on Catholic authority, not on the text of the Bible itself.
The rule of faith for Protestants is sola Scriptura. This belief holds that only Scripture is a final, infallible authority for the Christian: thus excluding the Church and Sacred Tradition as equally infallible authorities. Catholics and Protestants and Orthodox all happily agree that Holy Scripture is inspired. That’s not at issue.
The “conundrum” for Protestants, given their adherence to sola Scriptura, however, is how to “get” to Holy Scripture in the first place. The Bible never lists all of its own books (though there are indications of some of them, as we shall see). Thus, the Protestant is forced (rather frustratingly for them!) to a position where a major exception must be made for the sola Scriptura principle. The very Bible itself; it’s own contents and what it is, must be ultimately derived from what Protestants call an “unbiblical” or “extrabiblical” tradition: the authority of the early (Catholic) Church.
Prominent Presbyterian radio teacher R. C. Sproul has famously described this awkward scenario as “a fallible collection of infallible books.” He notes that some Protestants believe that there was a special protection of divine providence and the Holy Spirit in the very human, historical process of determining the canon. He grants the possibility of mistakes in the selection of books, yet he himself is confident that the right ones were selected.
It’s a very strange exception to the rule of usually confident Protestants, proclaiming that all their beliefs are “biblical” whereas so many Catholic doctrines supposedly contradict Scripture.
As a sort of thought-experiment, though, let’s see how much Scripture says about it’s own books. Some contend that when Jesus referred to “the law of Moses and the prophets and the psalms” (Lk 24:44, RSV), that this was a synonym for the 39-book Old Testament canon (minus the deuterocanon). We know that the Jews have categorized their Bible in a threefold sense of “Law, prophets, and writings.” That may very well be Jesus’ meaning.
More technically or specifically, we know that Jesus directly cited 24 Old Testament books, and that the New Testament cited from the “Protestant 39” all books except Ezra, Nehemiah, Esther, Ecclesiastes, and the Song of Solomon. But we shall grant for the sake of argument that Jesus recognized 39 books of the canon as understood by the Jews, apart from the seven disputed books of what Protestants call the “Apocrypha.”
It still remains, however, to determine the 27 New Testament books by the Bible itself. This is a much more difficult matter indeed. Protestants argue that the reference to St. Paul’s writings in 2 Peter 3:16 (“all his letters” and calling them “scriptures”) bears witness to his fourteen letters.
The problem, though, is that Peter doesn’t name the letters. There could have been more around than there are now. Paul himself refers to an earlier letter to the Corinthians, in his first epistle to them that we have in our Bibles (1 Cor 5:9). There may also have been a letter of his to the Laodiceans (see Col 4:16).
Or at the time of writing, Peter (even though his second epistle seems to have been written after all or most of Paul’s letters) may not have been aware of all the Pauline epistles that the Church at length decided to include in the New Testament. We simply don’t know. It’s pure speculation.
What we do know is that there were many “anomalies” as to the biblical books in the early Church. So, for example, the apostolic father St. Justin Martyr (d. 165) either didn’t cite or didn’t accept the canonicity of Philippians and 1 Timothy. He and St. Polycarp (d. 155) had the same view of 2 Timothy, Titus, and Philemon. Polycarp knew the apostle John.
We also know that St. Athanasius is the first known person to name all 27 New Testament books that are accepted today, in the year 367 AD: about eight biblical generations after the death of Jesus. Therefore, no one that we know of had the knowledge of all 27 of those books being part of the Bible, before 367.
Numerous books were disputed up until the fourth century: notably, Hebrews, James, Revelation, 2 and 3 John, and Jude, and also including 2 Peter: the very book that is cited as corroborating the Pauline corpus. The 1910 Catholic Encyclopedia, in its article on the “Epistles of St. Peter” chronicles the uncertainty about this book in the early Church:
In the first two centuries there is not in the Apostolic Fathers and other ecclesiastical writers, if we except Theophilus of Antioch (180), a single quotation properly so called from this Epistle . . . In the Western Church there is not explicit testimony in favour of the canonicity and Apostolicity of this Epistle until the middle of the fourth century. Tertullian and Cyprian do not mention it, . . . Eusebius of Caesarea (340), while personally accepting II Peter as authentic and canonical, nevertheless classes it among the disputed works (antilegomena), . . . St. John Chrysostom does not speak of it, . . .
One has to assume the unbiblical tradition that St. Peter's letters are Scripture, in order for them to (vaguely) attest to St. Paul's letters. Even after granting the book’s canonicity, we still don’t know for sure which Pauline books St. Peter was referring to, so this doesn’t solve the uniquely Protestant problem of a “Bible-determined canon.”
1 Timothy 5:18 appears to cite the Gospel of Matthew and call it “scripture.” That takes the argument a little further. But it will always be incomplete, by its very nature.
By sola Scriptura reasoning alone, Protestants should be radically uncertain as to the New Testament canon. But of course they do accept this canon, and it’s based on Catholic authority (fourth-century councils). Maybe they can be persuaded to like more Catholic traditions in due course. If the canon of the Bible is a “good” tradition, then maybe many other Catholic traditions are good, too!