A fortiori from the Jewish Church — Christ and Paul reject extra-Scriptural traditions
First argument: when Jewish Scripture was only the Old Testament, the whole salvation-doctrine was in Scripture, with no unwritten traditions equal to it.
A fortiori today: Scripture now includes the New Testament books that set out Christ's and the Apostles' teaching, so all the more must salvation-doctrine be contained in Scripture.
That Judaism had no equal-authority unwritten traditions follows because any such would have been the very traditions the Jewish teachers pressed — yet Christ shows those empty human inventions (Matthew 15; Mark 7).
When scribes and Pharisees (then the Jewish Church's doctors) charged the disciples with breaking the elders' tradition by unwashed hands (Matthew 15:2; Mark 7:5), Christ answered that they break God's commandment by their tradition and make God's word void (Matthew 15:3, 6; Mark 7:8-13).
He does not merely rebuke hypocrisy; He rejects their traditions as vain in general and draws no line between true and false extra-Scriptural traditions — citing Isaiah: This people honors Me with lips while the heart is far; in vain they worship Me, teaching human commandments (Isaiah 29:13 via Matthew 15:8-9; Mark 7:6-7).
Paul likewise warns against human tradition (Colossians 2:8) and recalls his own former zeal for ancestral traditions (Galatians 1:14).
So if Christ and the Apostles treat Jewish extra-Scriptural traditions as human doctrines, Rome's traditions fall the same way — they come with no greater authority.
Latin
XXVI. Primum ergo sic argumentamur. In Ecclesia Judaica, cum Scriptura sacra constaret solis Veteris Testamenti libris, tota salutis doctrina Scriptura continebatur, neque extabant ullae traditiones non scriptae aequalis cum Scriptura sacra authoritatis. Atqui multo magis credendum est totam salutis doctrinam hodie sacra Scriptura contineri, cum Scriptura jam aucta sit Novi Testamenti libris, in quibus exponitur doctrina Christi & Apostolorum. In Ecclesia vero Judaica non extitisse ullas traditiones non scriptas, ejusdem cum Scriptura sacra authoritatis, ex eo colligitur, quod, si quae tales habendae fuissent, proculdubio forent illae quas Judaeorum magistri commendabant, & quarum apud populum observationem urgebant. Sed illas vanas fuisse & hominum commenta docet Christus apud Matthaeum cap. 15. & apud Marcum cap. 7. Quum enim Scribae atque Pharisaei, qui tum erant Ecclesiae Judaicae Doctores, discipulos Christi arguerent, dicentes, Quare discipuli tui transgrediuntur traditionem seniorum? Non enim lavant manus quam panem edunt. Christus vicissim illos increpat, dicens, Quare & vos transgredimini praeceptum Dei per traditionem vestram? Item, irritum fecistis praeceptum Dei per vestram traditionem. Neque solum arguit eorum hypocrisin, quod mandatorum humanorum observationem urgerent, dum ipsi Dei mandata negligebant, & dogmatibus pravis corrumpebant: sed in genere declarat traditiones eorum tanquam frustraneas rejiciendas esse, nec ullas traditiones ut veras & legitimas a falsis & spuriis distinguit. Bene, inquit, de vobis prophetavit Isaias, sicut scriptum est, Populus iste labiis me honorat, cor autem eorum procul abest a me. Frustra vero me colunt docentes doctrinas quae sunt praecepta hominum. Nam omisso Dei praecepto tenetis traditiones hominum, lotiones sextariorum, & poculorum, aliaque similia hujusmodi multa facitis. Ideoque Paulus Coloss. 2. fideles monet ut sibi caveant ab hominum traditionibus, Videte, inquit, Ne quis vos decipiat per Philosophiam & inanem fallaciam, secundum traditionem hominum. Et Galat. 1. sibi ipsi vitio vertit quod ante suam ad Christum conversionem, Supra modum zelator fuerit paternarum traditionum. Cum ergo Christus & Apostoli damnent, ac pro doctrinis hominum habeant Judaeorum traditiones extra Scripturam sacram, pari ratione sequitur rejiciendas & damnandas esse traditiones Pontificiorum: cum illae non majori authoritate nobis commendentur quam Judaeorum olim traditiones.