"I answer that he is speaking here about keeping the commandments of the Law insofar as the Law consists of ceremonial precepts and moral precepts. This is the Law that is not of faith... Therefore, strictly speaking, he fulfills the command of faith who does not hope to obtain from it anything present and visible, but things invisible and eternal." (Commentary on Galatians 3:12; Aquinas Scripture Series, trans., F. R. Larcher, p. 83).
Galatians 3;10-12 is not the only place where Aquinas teaches that "works of the law' or "Law" in the New Testament refers to the whole law. In his commentary on Romans 3:20; 5:20; 7:6 and 2 Cor. 3:7 he says the same thing. Suffice it to say that in no place in his writings does Aquinas allow the idea of "works of the law = ceremonial law" to serve as the overriding concern in the analysis of how a man is justified. This is also true of the other prominent medieval theologians. I know of know one who used the "ceremonial law" apologetic.
Now we can see why the Council of Trent said what it did. The consensus of Church Fathers, including Augustine; the consensus of Medieval theologians, including Aquinas, made it quite clear that the "works of the law = ceremonial law" argumentation was at best a half-truth, and as such, it ended up distorting the Scripture's teaching on the important matter of Justification.
Lest anyone think that these views are from "traditional" Catholicism but that Catholics today have a deeper and better understanding on the issue of Justification, let me make it known that the 1992 Catholic Catechism takes the same approach as the Council of Trent. All the statements the Catechism gives on Justification (paragraphs 1987 through 2029) match, precept for precept, what the Catholic Church has traditionally taught on this subject. Not one time in those paragraphs does the 1992 Catechism use the argument that "works of the law" refer to the ceremonial law, to the exclusion of the moral law. Nor does the Catechism, when it is discussing the Law, make a distinction between the ceremonial and moral laws (paragraphs 1949 through 1986). Rather, everything I am teaching in this paper is taught in both the Council of Trent and the 1992 Catechism.
Let me also add the words of the Roman Catechism from the Council of Trent:
"But, lest the people, aware of the abrogation of the Mosaic Law, may imagine that the precepts of the Decalogue are no longer obligatory, it should be taught that when God gave the Law to Moses, He did not so much establish a new code, as render more luminous that divine light b which the depraved morals and longcontinued perversity of man had at that time almost obscured. It is most certain that we are not bound to obey the Commandments because they were delivered by Moses, but because they are implanted in the hearts of all, and have been explained and confirmed by Christ our Lord. The reflection that God is the author of the law is highly useful, and exercises great influence in persuading (to its observance); for we cannot doubt His wisdom and justice, nor can we escape His infinite power and might. Hence, when by His Prophets He commands the law to be observed, He proclaims that He is the Lord God; and the Decalogue itself opens: I am the Lord thy God; and elsewhere (we read): If I am a master, where is my fear? That God has deigned to make clear to us His holy will on which depends our eternal salvation (is a consideration) which, besides animating the faithful to the observance of His Commandments, must call forth their gratitude Hence Scripture, in more passages than one, recalling this great blessing, admonishes the people to recognise their own dignity and the bounty of the Lord Thus in Deuteronomy it is said: This is your wisdom and understanding in the sight of nations, that hearing all these precepts they may say: Behold a wise and understanding people, a great nation; again, in the Psalm (we read): He hath not done in like manner to every nation, and his judgments he hath not made manifest to them."
Scriptural Exegesis
Now let's address some of the passages that have been used by those advocating that "works of the law = ceremonial law."
We saw earlier that Romans 3:29 is often cited to support the idea that "works of the law" in Romans 3:28 refers to the ceremonial law. Here are those passages again:
28 For we maintain that a man is justified by faith apart from works of the Law. 29 Or is God the God of Jews only? Is He not the God of Gentiles also? Yes, of Gentiles also, 30 since indeed God who will justify the circumcised by faith and the uncircumcised through faith is one."
As noted earlier, it is argued that since the distinguishing characteristic between Jews and Gentiles is that the former practice the ceremonial law and the latter do not, then the "works of the Law" in 3:28 must refer to the ceremonial law. It is further argued that since the Jews and Gentiles have the moral law in common, then this leaves the ceremonial law as the only distinction between the Jew and Gentile.
"Works of the Law" - Page 5