Rome's 'Scripture cannot prove Scripture' does not defeat sufficiency
Here is how to meet the argument Rome loves: that Scripture is not a full and sufficient rule of faith because Scripture cannot be proved by Scripture.
A rule need not prove itself to be perfect and sufficient. It is enough that by it everything else of the same kind can be examined and proved.
In analyzing faith, unless you allow a circle or an infinite regress — both absurd — you must stop at some principle that cannot be proved by another better known in that order, and that cannot prove itself. Nothing proves itself.
Latin
XX. Atque hinc patet quomodo occurrendum sit argumento, quo se plurimum jactant Pontificii, dum, scilicet, se demonstrare putant Scripturam non esse plenam & sufficientem fidei regulam, quoniam ipsa Scriptura per Scripturam probari non potest. Nec enim, ut patet ex dictis, ad perfectionem & sufficientiam regulae requiritur, ut possit seipsam probare. Sed satis est ut per eam examinari & probari possint omnia praeter ipsam quae sunt ejusdem generis. Ac praeterea in fidei analysi, nisi quis velit admittere circulum, aut progressum in infinitum, quorum utrunque absurdum est, necesse est devenire ad principium quoddam in quo sistendum sit, & quod probari non possit per aliud in eo genere notius, nec etiam seipsum probare, quandoquidem, ut dictum est, nihil probat seipsum.