If the Church hangs on Scripture, Scripture cannot hang on the Church
How then could Scripture's authority for us hang on the Church's, when the Church's authority for us hangs on Scripture?
From Scripture we learn which Church is true, and what force it has. That authority cannot be proved with certainty from anywhere else. So the fathers once sent heretics who denied Christ's true Church back to the Scriptures, and held that the true Church must be shown from them. Augustine tells the Donatists: if they hold the Church, let them show it only from the canonical books of divine Scripture.
Rome itself today tries to prove its Church and its infallible authority against us from sacred Scripture, and thinks the work well done when it cites places such as "I am with you to the end of the age," "I have prayed for you, Peter, that your faith may not fail," "the Spirit will lead you into all truth," "Tell it to the Church," and the like, as foundations of the authority it gives the Church. Since, by their own confession, the Church's authority is known to us from Scripture, and no surer proof can be offered, it is clear the other way that our certainty about Scripture does not rest on the Church's testimony as its chief ground. Otherwise the circle is open: Scripture is proved by the Church, the Church by Scripture, and back again without end.
Latin
XXXII. Quomodo vero posset Scripturae authoritas quoad nos ab authoritate Ecclesiae pendere, cum contra, authoritas Ecclesiae quoad nos ab authoritate Scripturae pendeat. Etenim ex Scriptura discimus quae sit vera Ecclesia, et quae et quanta sit illius authoritas. Nec authoritas illa potest aliunde certo probari. Ideoque patres olim ad Scripturas revocarunt haereticos qui veram Christi Ecclesiam non agnoscebant, et ex iis veram Ecclesiam demonstrandam esse censuerunt. Sic enim Augustinus cum Donatistis agit: Utrum ipsi, inquit, Ecclesiam teneant nonnisi divinarum Scripturarum canonicis libris ostendant. Et ipsi Pontificii hodie Ecclesiam suam, et authoritatem ejus infallibilem, contra nos ex Scriptura sacra probare conantur, et eo se officio egregie defungi putant dum nobis afferunt illa Scripturae loca, Ero vobiscum usque ad consummationem saeculi, Rogavi pro te, Petre, ne fides tua deficiat, Spiritus vos deducet in omnem veritatem, Dic Ecclesiae, et similia quaedam quae proferunt tanquam fundamenta authoritatis illius quam Ecclesiae tribuunt. Cum igitur, ipsis fatentibus, ex Scriptura authoritas Ecclesiae nobis innotescat, nec certior possit ejus afferri probatio, planum est vice versa certitudinem nostrae fidei de Scriptura, Ecclesiae testimonio, tanquam praecipuo suo fundamento, non niti. Alioquin hic admittetur circulus. Nam Scripturae authoritas per Ecclesiam probabitur, et vicissim authoritas Ecclesiae per Scripturam; Et rursus a Scriptura ad Ecclesiam redibitur, et sic in infinitum.