Rome's Church-testimony claim collapses by the same standard they press against Scripture
Whichever way Rome turns, it must confess that the Church's testimony — which it offers as faith's first and supreme principle — is not evident of itself, and cannot well be proved by the Church's own voice. Yet they still want that testimony to hang on no prior authority and to hold among the faithful of itself. They fight themselves when, from Scripture's inability to prove Scripture, they conclude that Scripture's authority is not first and needs a superior testimony to establish it for us.
The sum of our reply comes to this: Scripture cannot indeed be proved by Scripture itself; yet it does not follow that its authority hangs elsewhere and must be proved to us by the Church as a principle prior and superior to Scripture. As faith's first principle, Scripture wins the faithful's belief and assent not by such a proof intervening, but immediately, by the Spirit's inward efficacy impressing a sure persuasion of it on our minds.
Latin
VII. Et certe quocunque se vertant Pontificii, omnino fateri cogentur Ecclesiae testimonium, quod tanquam primum et supremum fidei principium obtendunt, non esse rem per se evidentem, nec per ipsam Ecclesiae vocem et testimonium idonee posse probari. Cum ergo nihilominus Ecclesiae testimonium ab alia nulla priori authoritate pendere, sed per se suam authoritatem apud fideles obtinere velint, aperte contra seipsos pugnant, quum ex eo quod Scriptura per Scripturam probari non possit, concludunt authoritatem illius non esse primam, et illam superiori indigere testimonio, quo nobis comprobetur. Huc ergo redit summa responsionis nostrae, Scripturam quidem per Scripturam ipsam probari non posse; non tamen inde sequi authoritatem ejus aliunde pendere, et nobis per authoritatem Ecclesiae debere probari, tanquam per quoddam fidei principium Scriptura ipsa prius et superius: quoniam Scriptura, tanquam primum fidei principium, fidem et assensum apud fideles obtinet, non probationis talis interventu, sed immediate per internam Spiritus efficaciam certam de illa persuasionem mentibus nostris imprimentis.