IX. The infallible rule of interpretation of Scripture is the Scripture itself: and therefore, when there is a question about the true and full sense of any Scripture (which is not manifold, but one), it must be searched and known by other places that speak more clearly.
This is a succinct statement of the “analogy of faith” or “analogy of Scripture,” which for Protestants are of course extremely closely related (there is a sense in which they could be taken as identical, and a sense in which a creed or confession [rather than the Bible itself] could serve a “rule of faith,” a regula or kanon, against which doctrinal constructions should be compared and judged [analogia]–but only because, and in so far as, these creeds or confessions are themselves faithful summaries of what the Bible teaches, against which they themselves must be judged).
And once again one can find statements in Augustine (e.g.) that point in this direction, such that the Protestant Scripture principle can be viewed as a kind of “suppressed tradition” within the broader reception of the Christian faith in history.
8. But why I view them with greater delight under that aspect than if no such figure were drawn from the sacred books, though the fact would remain the same and the knowledge the same, is another question, and one very difficult to answer. Nobody, however, has any doubt about the facts, both that it is pleasanter in some cases to have knowledge communicated through figures, and that what is attended with difficulty in the seeking gives greater pleasure in the finding.— For those who seek but do not find suffer from hunger. Those, again, who do not seek at all because they have what they require just beside them often grow languid from satiety. Now weakness from either of these causes is to be avoided. Accordingly the Holy Spirit has, with admirable wisdom and care for our welfare, so arranged the Holy Scriptures as by the plainer passages to satisfy our hunger, and by the more obscure to stimulate our appetite. For almost nothing is dug out of those obscure passages which may not be found set forth in the plainest language elsewhere.
Just in case you stumble over that word “almost”–“See, he doesn’t say that absolutely everything is found clearly expressed in the plainer passages, therefore tradition! magisterium! Etc.!”–fret not. One can employ the analogia Augustini and find a place where he perhaps speaks even more, ahem, clearly. This he does in On Merit and the Forgiveness of Sins 3.7:
Now, although I may not be able myself to refute the arguments of these men, I yet see how necessary it is to adhere closely to the clearest statements of the Scriptures, in order that the obscure passages may be explained by help of these, or, if the mind be as yet unequal to either perceiving them when explained, or investigating them while abstruse, let them be believed without misgiving. But what can be plainer than the many weighty testimonies of the divine declarations, which afford to us the clearest proof possible that without union with Christ there is no man who can attain to eternal life and salvation; and that no man can unjustly be damned,— that is, separated from that life and salvation—by the judgment of God? The inevitable conclusion from these truths is this, that, as nothing else is effected when infants are baptized except that they are incorporated into the church, in other words, that they are united with the body and members of Christ, unless this benefit has been bestowed upon them, they are manifestly in danger of damnation. Damned, however, they could not be if they really had no sin. Now, since their tender age could not possibly have contracted sin in its own life, it remains for us, even if we are as yet unable to understand, at least to believe that infants inherit original sin.
One corollary of this is that the principle can be applied to Augustine’s own understanding of the Scriptures (as he himself would have wanted); and to put a somewhat fine point on it, one could do so in the very sentences that follow his general principle: is it really the case that Scripture is clear that unbaptized infants are “manifestly in danger of damnation” if, e.g., they should perish without it? In other words, is salvation so closely annexed to it that we must be extremely uncertain of the status of anyone who dies without it? The answer, not to beat around the bush, is “obviously not.” One can agree that the “divine declarations” teach that “without union with Christ there is no man who can attain to eternal life and salvation” and disagree with Augustine’s (putatively) “inevitable conclusion” that he draws from it. That is not my purpose or my point here (for a discussion of Augustine’s view, cf. this post); the point is that the analogy of Scripture is precisely what allows one the freedom to have that debate. Both sides must be on an equal footing, equally liable to have their pronouncements judged at the tribunal of the Word of God, for God is the only one who can say what must be belived. The matter in view is one of principle rather than the application of that principle: which is to say, Augustine may give voice to the right principle, even if a strong case could be made that he misapplies that principle in particular instances. But the principle is what is important; and Protestants of the sixteenth and seventeenth centuries found expressions of it going back to antiquity.