Here's an English translation of the relevant portion:
It should be noted that though many might write concerning Catholic truth, there is this difference that those who wrote the canonical Scripture, the Evangelists and Apostles, and others of this kind, so constantly assert it that they leave no room for doubt. That is his meaning when he says ‘we know his testimony is true.’ Galatians 1:9, “If anyone preach a gospel to you other than that which you have received, let him be anathema!” The reason is that only canonical Scripture is a measure of faith. Others however so wrote of the truth that they should not be believed save insofar as they say true things.Latin text:
Notandum autem, quod cum multi scriberent de catholica veritate, haec est differentia, quia illi, qui scripserunt canonicam Scripturam, sicut Evangelistic et Apostoli, et alii huiusmodi, ita constanter eam asserunt quod nihil dubitandum relinquunt. Et ideo dicit Et scimus quia verum est testimonium eius; Gal. I, 9: Si quis vobis evangelizaverit praeter id quod accepistis, anathema sit. Cuius ratio est, quia sola canonica scriptura est regula fidei. Alii autem sic edisserunt de veritate, quod nolunt sibi credi nisi in his quae ver dicunt.”And here's the citation: Thomas Aquinas, Lectures on the Gospel of John, ed. P. Raphaelis Cai, O.P., Editio V revisa (Romae: Marietti Editori Ltd., 1952) n. 2656, p. 488.
This is not an attempt to construe Aquinas as a modern-day Reformed believer (or even a "Protestant" as unhelpful as that category is). Such an allegation would be anachronistic. However, this citation does show that it is equally (if not more-so) anachronistic to view Aquinas as sharing the beliefs of modern-day Roman apologists. In short, his view of Scripture may not be precisely the same as ours, but it is also not the same as that of Rome, in an important way.
The usual response to this sort of citation from Aquinas is exemplified by the response provided by Phil Porvaznik (link) who deflects from the text in question to another place in Aquinas' writings that he thinks is inconsistent with Sola Scriptura. While such an approach may help to prove what we already concede (namely that Aquinas is not simply a modern-day Reformed Presbyterian), it does not answer the crucial question, what did Aquinas mean by "sola canonica scriptura est regula fidei" ("only canonical scripture is [the/a (Latin lacks articles)] rule of faith")? Can any of the Roman Catholics reading answer that question positively (i.e. by refraining from telling us what Aquinas is not saying but rather by telling us what Aquinas is saying)?
What is interesting is that this is not the only time Aquinas speaks of the rule of faith. Here's another place, first an English translation:
Or, wanting to show those speeches that are completely outside of the Scriptures, it said: If they will say to you: Here, and in the desert, do not depart from the rule of the faith.Latin text:
Vel eos sermones qui sunt omnino extra Scripturam ostendere volens, dixit si dixerint vobis: ecce in solitudine est, nolite exire, de regula fidei.Citation: Thomas Aquinas, Catena Aurea, Commentary on the Gospels, at Matthew 24:23-38 (Lectio 6 in Matthew 24), quoting (apparently with approval) from Origen.
Additionally, we may note another such reference (again English first):
Objection 1. It would seem that it is unsuitable for the articles of faith to be embodied in a symbol. Because Holy Writ is the rule of faith, to which no addition or subtraction can lawfully be made, since it is written (Deuteronomy 4:2): "You shall not add to the word that I speak to you, neither shall you take away from it." Therefore it was unlawful to make asymbol as a rule of faith, after the Holy Writ had once been published.Latin text:
Reply to Objection 1. The truth of faith is contained in Holy Writ, diffusely, under various modes of expression, and sometimes obscurely, so that, in order to gather the truth of faith from Holy Writ, one needs long study and practice, which are unattainable by all those who require to know the truth of faith, many of whom have no time for study, being busy with other affairs. And so it was necessary to gather together a clear summary from the sayings of Holy Writ, to be proposed to the belief of all. This indeed was no addition to Holy Writ, but something taken from it.
Ad nonum sic proceditur. Videtur quod inconvenienter articuli fidei in symbolo ponantur. Sacra enim Scriptura est regula fidei, cui nec addere nec subtrahere licet, dicitur enim Deut. IV, non addetis ad verbum quod vobis loquor, neque auferetis ab eo. Ergo illicitum fuit aliquod symbolum constituere quasi regulam fidei, post sacram Scripturam editam.Citation: Thomas Aquinas, Summa Theologica, Second Part of the Second Part, Question 1, Article 9
Ad primum ergo dicendum quod veritas fidei in sacra Scriptura diffuse continetur et variis modis, et in quibusdam obscure; ita quod ad eliciendum fidei veritatem ex sacra Scriptura requiritur longum studium et exercitium, ad quod non possunt pervenire omnes illi quibus necessarium est cognoscere fidei veritatem, quorum plerique, aliis negotiis occupati, studio vacare non possunt. Et ideo fuit necessarium ut ex sententiis sacrae Scripturae aliquid manifestum summarie colligeretur quod proponeretur omnibus ad credendum. Quod quidem non est additum sacrae Scripturae, sed potius ex sacra Scriptura assumptum.
1. It seems that you should not combine the articles in the symbol. In fact all the faith is taught in a comprehensive manner with Sacred Scripture. So it was unnecessary to compose the symbol.Latin Text:
2. The symbol is proposed as a rule of faith, whose action is consent. Now, only the Apostles and Prophets must be granted this honor, which is that all they have said is believed to be true, as St. Augustine asserts. So after the Apostles' Creed one should not draw up other symbols.
Reply to 1. It was necessary to collect in a single text the various truth transmitted in various places of the Sacred Scriptures so that the faith would be more readily at hand.
Reply to 2. The Fathers who have published other symbols after the Apostles have not added anything of their own, but added what they excerpted from the Holy Scriptures. Now, since in that symbol of the Apostles there are some difficult things, the Nicene Creed was published, which exposes more fully the faith about certain items. Since then some truths were contained in those symbols in implicit form, it was necessary to give an explanation upon the rise of heresies, and so was added the symbol S. Athanasius, who especially set himself against the heretics.
Ulterius. Videtur quod articuli non debuerunt colligi in symbolo. Quia tota fides sufficienter per sacram Scripturam instruitur. Ergo superfluum fuit symbolum condere.Citation: Thomas Aquinas, Commentary upon [Lombard's] Sentences, Book 3, Distinction 25, Question 1, Answer 1, Quaestiuncula 3, arguments 1-2 and answer to arguments 1-2
Praeterea, symbolum proponitur ut regula fidei, cujus actus est assentire. Sed, sicut dicit Augustinus in epistola 19 ad Hieronymum, solis apostolis et prophetis est hic honor exhibendus, ut quaecumque dixerunt, haec ipsa vera esse credantur. Ergo post symbolum apostolorum non debuerunt alia symbola fieri.
Ad primum ergo dicendum, quod oportuit ea quae in diversis locis sacrae scripturae tradita sunt, in unum colligi locum, ut fides magis in promptu haberetur.
Ad secundum dicendum, quod patres qui alia symbola post apostolos ediderunt, nihil de suo apposuerunt; sed ex sacris scripturis ea quae addiderunt, exceperunt. Et quia quaedam difficilia sunt in illo symbolo apostolorum, ideo ad ejus explanationem editum est symbolum nicaenum, quod diffusius fidem quantum ad aliquos articulos prosequitur. Et quia quaedam implicite continebantur in illis symbolis, quae oportebat propter insurgentes haereses explicari; ideo additum est symbolum athanasii, qui specialiter contra haereticos se opposuit.
4. In the symbol, the faith must be exposed that extends to all believers. But, not all believers have come to believe in God, but only those who have a formed faith. Therefore he seems to say badly: “I believe in a single God,” and because he has a shapeless faith, saying this, sins by lying.Latin Text:
Reply to 4. In the symbol is propounded to us the rule of the faith, to which all must come. But they do not have only to reach the action of shapeless faith, but also the action of formed faith. However, he who, having shapeless faith, recites the symbol, does not sin, because he says this in the person of the Church.
Praeterea, in symbolo debet exponi fides quantum ad omnes credentes. Sed non omnibus credentibus convenit credere in Deum, sed tantum habentibus fidem formatam. Ergo videtur quod male dictum sit: credo in unum Deum; et quod habens fidem informem, hoc dicens peccet mentiendo.Citation: Thomas Aquinas, Commentary upon [Lombard's] Sentences, Book 3, Distinction 25, Question 1, Answer 2, Argument 4, and reply to 4
Ad quartum dicendum, quod in symbolo proponitur nobis regula fidei, ad quam omnes debent pertingere. Non autem debent pertingere solum ad actum fidei informis, sed etiam ad actum fidei formatae, et ideo ponitur in symbolis actus fidei formatae. Nihilominus habens fidem informem, dicens symbolum, non peccat: quia hoc dicit in persona Ecclesiae.
3. The Sacred Scripture is the rule of the faith. But, in the Scriptures of Old Testament the Trinity was not explicitly mentioned. Therefore it was not necessary [to believe in the Trinity] in order to believe.Latin Text:
Reply to 3. Since it was not necessary that all be explicitly known in the Old Testament, the mystery of the Trinity was not formulated manifestly, but veiled, so that the wise can understand.
Praeterea, sacra Scriptura est regula fidei. Sed in Scriptura veteris testamenti non fuit mentio expressa facta de Trinitate. Ergo non erat necessaria ad credendum.Citation: Thomas Aquinas, Commentary upon [Lombard's] Sentences, Book 3, Distinction 25, Question 1, Answer 2, Quaestiuncula 4, Argument 3, and reply to 3
Ad tertium dicendum, quod quia non erat necessarium ut explicite omnes cognoscerent, ideo non fuit positum mysterium trinitatis manifeste in veteri testamento, sed velate ut sapientes capere possent.
Finally, again, first in English:
Objection 3. Further, Athanasius was not the Sovereign Pontiff, but patriarch of Alexandria, and yet he published a symbol which is sung in the Church. Therefore it does not seem to belong to the Sovereign Pontiff any more than to other bishops, to publish a new edition of the symbol.Latin text:
Reply to Objection 3. Athanasius drew up a declaration of faith, not under the form of a symbol, but rather by way of an exposition of doctrine, as appears from his way of speaking. But since it contained briefly the whole truth of faith, it was accepted by the authority of the Sovereign Pontiff, so as to be considered as a rule of faith.
Praeterea, Athanasius non fuit summus pontifex, sed Alexandrinus patriarcha. Et tamen symbolum constituit quod in Ecclesia cantatur. Ergo non magis videtur pertinere editio symboli ad summum pontificem quam ad alios.Citation: Thomas Aquinas, Summa Theologica, Second Part of the Second Part, Question 1, Article 10
Ad tertium dicendum quod Athanasius non composuit manifestationem fidei per modum symboli, sed magis per modum cuiusdam doctrinae, ut ex ipso modo loquendi apparet. Sed quia integram fidei veritatem eius doctrina breviter continebat, auctoritate summi pontificis est recepta, ut quasi regula fidei habeatur.
Recall again that the challenge to the Roman Catholic reader is to tell us what Aquinas meant by saying that the canonical scriptures alone are a/the rule of faith.
To make the Roman Catholic's job easier, here are some important negative points that I'll present so that the Roman Catholic can focus on the positives:
1. Aquinas wrote in Latin, so while we might be tempted to insert "the" before "rule of faith," the sense of "the" can only be implied.
2. In the first quotation above, the Scriptures are not being contrasted with the proclamations of ecumenical councils or ex cathedra papal statements (the latter category wasn't really yet in existence in Aquinas' time). Thus, Aquinas is not specifically and directly speaking to the supremacy of Scripture over conciliar and papal documents, as such.
Finally, here are some additional quotations from Aquinas, which - while they don't expressly use the expression "rule of faith" -- help to inform the discussion.
First, some explanation of what the expression "canonical" with reference to Scripture meant to Aquinas:
If you wish to know whether a doctrine be erroneous, he shows this by three things. First, if it be against ecclesiastical doctrine. And therefore he says, If any man teach otherwise, namely, than I or the other Apostles. Gal. 1:9: If any one preach to you a gospel, besides that which you have received, let him be anathema. For the doctrine of the Apostles and prophets is called canonical, since it is like a rule for our intellect. And therefore no one ought to teach otherwise. Deut. 4:2: You shall not add to the word that I speak to you, neither shall you take away from it. Apoc. 22:18: If any man shall add to these things, God shall add unto him the plagues written in this book.
Regarding the second he says, and consent not, etc. For the Lord Jesus came to give testimony to the truth. Jn. 18:37: For this I was born, and for this came I into the world; that I should give testimony to the truth. And therefore He was sent by the Father as a doctor and teacher. 1 Mach. 2:65: Give ear to him always, and he shall be a father to you, etc. And therefore whatever does not conform to their words is erroneous. 1 Kg. 15:23: It is like the sin of witchcraft, to rebel: and like the crime of idolatry, to refuse to obey. And he says, sound, because in the words of Christ nothing is corrupt, nothing false, or perverse, since they are words of divine wisdom. Prov. 8:8: All my words are just, there is nothing wicked nor perverse in them. They are right to them that understand, and just to them that find knowledge.Latin text:
Regarding the third, it says in Prov. 6:20, My son, keep the commandments of thy father, and forsake not the law of thy mother. Whence he says, and to that doctrine which is according to godliness, namely, ecclesiastical doctrine. This godliness is through the worship of God. Tit. 1:1: According to…the acknowledging of the truth, which is according to godliness.
Si vis scire, quae doctrina sit erronea, hoc ostendit ex tribus. Primo si sit contra doctrinam ecclesiasticam. Et ideo dicit si quis aliter docet, scilicet quam ego et alii apostoli, quantum ad primum. Gal. I, 9: si quis vobis evangelizaverit praeter id quod accepistis, anathema sit. Doctrina enim apostolorum et prophetarum dicitur canonica, quia est quasi regula intellectus nostri. Et ideo nullus aliter debet docere. Deut. Iv, 2: non addetis ad verbum quod loquor vobis, neque auferetis ex eo. Apoc. Ult.: si quis apposuerit ad haec, apponet deus super illum plagas scriptas in libro isto.
Quantum ad secundum dicit et non acquiescit, etc.. Nam dominus iesus venit, ut testimonium perhibeat veritati. Io. Xviii, 37: in hoc natus sum, et ad hoc veni in mundum, ut testimonium perhibeam veritati. Et ideo missus est a patre sicut doctor et magister. i mac. Ii, 65: ipsum audite semper, et ipse erit vobis pater, etc.. Et ideo erroneus est quicumque non acquiescit sermonibus eius. I reg. Xv, 23: quasi peccatum ariolandi est repugnare, et quasi scelus idololatriae nolle acquiescere. Et dicit sanis, quia in christi sermonibus nihil est corruptionis, nihil falsitatis, vel perversitatis, quia sunt sermones divinae sapientiae. Prov. Viii, 8 s.: iusti sunt sermones mei, non est in eis pravum quid neque perversum. Recti sunt intelligentibus, et aequi invenientibus scientiam. quantum ad tertium, prov. Vi, 20: conserva, fili mi, praecepta patris tui, et ne dimittas legem matris tuae. Unde dicit et ei quae secundum pietatem est doctrinae, scilicet ecclesiasticae. Haec pietas est per cultum dei. Tit. I, 1: secundum agnitionem veritatis, quae est secundum pietatem.Citation: Thomas Aquinas, Commentary on 1 Timothy, Chapter 6, Lecture One.
Thomas Aquinas similarly asserts that Scripture's ultimate effect is to lead men to perfection, by teaching men not only the things necessary for salvation but also for supererogation (see, there's an example of Thomas not being a Reformed theologian, we deny that there are such things as works of supererogation). The text is interesting at least from the standpoint of material sufficiency:
Its ultimate effect is that it leads men to perfection. For it does good not in whatever manner, but it perfects. Heb. 6:1: Let us go on to things more perfect. And so he says, That the man of God may be perfect, since a man cannot be perfect unless he is a man of God. For something is perfect which lacks nothing. Therefore, then is a man perfect when he is furnished, that is, prepared, to every good work, not only for those which are necessary for salvation but also for those which are of supererogation. Gal. 6:9: And in doing good, let us not fail.Latin Text:
Ultimus eius effectus est, ut perducat homines ad perfectum. Non enim qualitercumque bonum facit, sed perficit. Hebr. C. Vi, 1: ad perfectionem feramur. Et ideo dicit ut perfectus sit homo dei, quia non potest homo esse perfectus, nisi sit homo dei. Perfectum enim est, cui nihil deest. Tunc ergo homo est perfectus, quando est instructus, id est, paratus, ad omne opus bonum, non solum ad ea quae sunt de necessitate salutis, sed etiam ad ea quae sunt supererogationis. Gal. Cap. Ult.: bonum autem facientes, non deficiamus.Citation: Thomas Aquinas, Commentary on 2 Timothy, Chapter 3, Lecture three.
Finally, Aquinas that while other arguments from probability can be made, the proper authority for holy teaching is the canonical Scripture:
Yet holy teaching employs such authorities only in order to provide as it were extraneous arguments from probability. Its own proper authorities are those of canonical Scripture, and these it applied with convincing force. It has other proper authorities, the doctors of the Church, and these it looks to as its own, but for arguments that carry no more than probability.Latin text:
For our faith rests on the revelation made to the Prophets and Apostles who wrote the canonical books, not on a revelation, if such there be, made to any other teacher. In this sense St Augustine wrote to St Jerome; Only to those books or writings which are called canonical have I learnt to pay such honour that I firmly believe that none of their authors have erred in composing them. Other authors, however, I read to such effect that, no matter what holiness and learning they display, I do not hold what they say to be true because those were their sentiments.
Sed tamen sacra doctrina hujusmodi auctoritatibus utitur quasi extraneis argumentis et probabilibus. Auctoritatibus autem canonicae Scripturae utitur proprie, ex necessitate argumentando. Auctoritatibus autem aliorum doctorum Ecclesiae, quasi arguendo ex propriis, sed probabiliter.Citation: Thomas Aquinas, Summa Theologica, Part 1, Question 1, Article 8
Innititur enim fides nostra revelationi apostolis et prophetis factae qui canonicos libros scripserunt, non autem revelationi, si qua fuit, aliis doctoribus factae. Unde dicit Augustinus in epistola ad Hieronymum; Solis eis Scripturarum libris qui canonici appellantur didici hunc honorem deferre, ut nullum auctorem in scribendo errasse aliquid firmissime credam. Alios autem ita lego ut, quantalibet sanctitate doctrinaque praepolleant, non ideo verum putem quod ipsi ita senserunt.
Any takers for the challenge? (Remember, the challenge is to tell us what Aquinas does mean, not what he doesn't mean.)