What cannot be demonstrated can still be a sure rule of faith
Next the argument assumes that what cannot be demonstrated by necessary and evident proofs cannot be certain for faith, or a sure rule and foundation of faith among believers. That is utterly false.
Whatever first rule of faith you set must be something that cannot be proved by evident — and, speaking logically, strictly necessary — arguments.
Such proofs would have to come from its own testimony, from other principles of faith, or from human reason. It cannot prove itself from its own witness. It cannot be proved from other faith-principles, because as first rule it is already supposed the best-known principle — so weaker ones would be useless except perhaps ad hominem. And it cannot be proved from human reason: a rule of faith is properly inevident, and so can only be urged by moral arguments, not demonstrated by necessary ones.
Latin
XXI. Deinde supponitur in isto argumento id quod demonstrari non potest, & necessariis ac evidentibus argumentis probari, non posse esse fidei certum & exploratum, certamque apud fideles fidei regulam atque fundamentum. At hoc est falsissimum. Etenim prima fidei regula, quaecunque tandem statuatur, necesse omnino est ut sit aliquid, quod evidentibus, &, si logice loquamur, prorsus necessariis argumentis probari non potest. Siquidem argumenta illa, vel ex proprio ipsius testimonio peterentur, vel ex aliis fidei principiis, vel ex humana ratione. Jam vero peti non possunt ex ipsius testimonio, quoniam hoc modo nihil probat seipsum: non etiam ex aliis fidei principiis, quia quod ponitur esse prima fidei regula hoc ipso supponitur esse principium fidei notissimum, ad quod proinde probandum, alia minus nota frustra adhiberentur; nisi forte ad hominem: denique non ex humana ratione; quia, cum regula fidei sit aliquid proprie loquendo inevidens, non potest rationibus necessariis demonstrari, sed solum argumentis moralibus suaderi.