Thomas on Scripture as the incontrovertible proof
Thomas defines three layers of theological arguments. At the first layer, we can use authorities like philosophers. But such arguments are extrinsic to the faith and only probable.
Second, we can use the arguments of church doctors (teachers). While these arguments are intrinsic to the faith, they also remain probable.
Third, we can use the only “incontrovertible proof” which is the “authority of the canonical Scriptures.”
He concludes, “For our faith rests upon the revelation made to the apostles and prophets who wrote the canonical books, and not on the revelations (if any such there are) made to other doctors” (ST bk I pt 1, 1.8).
His point, if I understand him, is this: Scriptural revelation must be believed since it has divine authority; other sources of truth only provide probable truths. Scripture alone gives incontrovertible truths, or proofs.
Elsewhere, he writes: “Only the canonical Scriptures are normative for faith” (Joan. 21, Lect. 6, qtd in Lamb 1966: 19).
With that said, Protestants might want to rephrase what he says here. But it’s not exactly far from the truth.
Thomas on inerrancy
Thomas immediately quotes Augustine affirmingly to explain how he understands scriptural authority:
“Only those books of Scripture which are called canonical have I learned to hold in such honor as to believe their authors have not erred in any way in writing them. But other authors I so read as not to deem everything in their works to be true, merely on account of their having so thought and written, whatever may have been their holiness and learning” (Epis. ad Hieron. xix, 1 cited in ST bk I pt 1, 1.8).
Note: Thomas approvingly cites Augustine who claims: (1) Scriptural authors “have not erred” (inerrancy) and (2) other authors need to be critically read despite being holy or learned.
Scripture is utterly unique for both men. It is inerrant, authoritative, and a first principle upon which incontrovertible proofs stand.
Thomas also says things that Protestants would strongly oppose (on Mary, on the sacraments, and so on). Yet enough continuities exist to show a progression in history at the Reformation rather than a clean break (many more such examples could be given).
The particular debates of 16th-century theologians should not be imposed upon 13th or 9th century Christians. We must seek to understand authors in their own time and idiom.
Imputation rightly specifies the doctrine of justification. Yet before this doctrine received its specification at the Reformation, Christians like Gottshalk or Bernard of Clairvaux still faithfully served during their time, lamentably with worts and wrinkles (as we today have).
If we can be certain of one thing, it is that Christ will build his church. Yes, lapses happen. Yes, doctrine sometimes gets waylaid. But in the end, Christ is building it. Let’s pray for a continual reformation.