It is a commonplace of Catholic theology that divine revelation consists in both the written (Scripture) and unwritten tradition handed on by the apostles to the stewardship of the Church (cf. Dei Verbum, Sec. 7-8). Classing the latter category with the former is bound to raise Protestant hackles — isn’t this just a transparent bid to exalt the later inventions of men to the same status as the LORD’s biblical word?
The first thing to note is that the decree actually recognizes a privileged status for Scripture over against the unwritten tradition: “the apostolic preaching, which is expressed in a special way in the inspired books, was to be preserved by an unending succession of preachers until the end of time” (Dei Verbum, Sec. 8). The only words whose liturgical presentation can be marked as “the Word of the LORD” are to be found in Scripture.
Nonetheless, if the twofold OT and NT are jointly to constitute the LORD’s scriptural revelation to us, there must be at least one piece of information revealed outside this textual corpus, namely, the extent of that corpus itself. That is, no part of either OT or NT specifies the books which jointly constitute the OT and NT, and yet that specification must itself be a matter of revelation. The alternative would be tantamount to the LORD’s saying, “I’ve revealed my will for you in a number of books, but I won’t tell you which ones.” But that would amount to the same as there not being any revelation in the first place, which is absurd.
The extent of the biblical canon is exactly an unwritten revelation, one acknowledged (at least implicitly) by all Christians who recognize the unique authority of Scripture as the LORD’s Word. Far from being an unbiblical notion, the authority of unwritten tradition is an entailment of the doctrine of biblical inspiration.