Formalized, the major is false: a first principle need not prove itself or be evident
To expose the cheat, reduce the argument to form. They must argue: whatever is not evident of itself and cannot prove itself is not a first principle of faith or supreme rule whose authority does not hang elsewhere — it must be proved through something else. But Scripture is not evident, nor can it prove itself. Therefore, and so on.
The major is plainly false. On the contrary: what is a first principle of faith must neither prove itself nor be proved through another — and yet need not be something evident. In the human sciences some principles are first: the rest hang on them and are proved through them, while they hang on nothing else in that order, or they would not be first, and win assent of themselves. Faith is the same.
Either admit a circle, or an infinite regress — both absurd — or stop at some principle on which the rest of faith hangs, proved by no prior principle and not by itself either, since the same thing cannot prove itself, yet winning immediate belief and assent of itself.
Latin
IV. Ut autem hujus argumenti fraus et vitium detegatur, ad formam revocandum est. Sic igitur necesse est ut illi argumententur. Quod ex se non est evidens, nec seipsum probare potest, illud non est primum fidei principium, et suprema fidei regula, cujus authoritas aliunde non pendet, sed ipsum per aliud probari necesse est. At Scriptura non est aliquid evidens, nec seipsam probare potest. Ergo, &c. Jam vero major hujus argumenti est falsi manifesta. Nam plane contra necesse est, ut quod est primum fidei principium, neque seipsum probet, nec per aliud probetur; nec tamen sit aliquid evidens. Etenim quemadmodum in disciplinis humanis quaedam sunt prima principia, unde reliqua pendent, et per quae caetera probantur, ipsa tamen neque a se, neque ab aliis in hoc genere pendent, alioqui prima non essent, sed per se fidem et assensum obtinent: ita quoque res se habet in fidei doctrina. Vel enim debet hic admitti circulus, aut dari progressus in infinitum, quorum utrumque est absurdum; aut necessario sistendum est in principio aliquo, unde pendeant, et per quod probentur caetera quae ad fidem pertinent, ipsum tamen a nullo priori, scilicet, probetur, neque etiam a seipso: quandoquidem idem non potest a seipso probari: sed per se fidem et assensum obtineat, et cui immediate credamus et assentiamur.