Unfortunately, the meaning of "works of the law" is such a crucial area to understanding both St. Paul and the Catholic teaching on Justification, that I feel compelled to reiterate more forcefully what I have already written in my 1997 book, Not By Faith Alone.
Various Catholic apologists today, when teaching on the meaning of the "works of the law," will often explain it as referring to the ceremonial law of Israel, to the exclusion, or the virtual exclusion, of the remaining law in Israel. (The ceremonial law refers to all the ritual religious practices, such as circumcision, eating kosher foods, priestly sacrifices, seventh-day sabbath observance, etc).
Sad to say, that answer is at best a half-truth, and at worst, it is a distortion of the Catholic teaching on Justification.
One of the reasons these apologists categorize "works of the law" as referring to the ceremonial law is that they have found it to be an easy polemical tool against Protestants. Protestants say that St. Paul condemns ALL work as having any part in Justification. The Catholic apologist counters by saying that when Paul uses the phrase "works of the law" he does not mean ALL works; he only means the works of the ceremonial law of Israel.
The Catholic will then add that in Paul's confining "works of the law" to the ceremonial law, he specifically meant to exclude the moral law, such as those we find in the Commandments. Therefore, in Romans 3:28, Paul really means: "For we maintain that a man is justified by faith apart from works of the Ceremonial Law," (but all other "good" works can, and do, justify a man).
In giving this kind of answer, the Catholic thinks he has satisfactorily defended the Catholic faith and silenced the Protestant. To bolster his case, he may enlist the help of Romans 3:29 as proof that his answer is correct: "Or is God the God of Jews only? Is He not the God of Gentiles also? Yes, of Gentiles also." He then explains that since Paul speaks of "Jews only," then the "works of the law" mentioned in the previous verse (3:28) must be something that identifies only with the Jews but not with the Gentiles. In that he is correct, but as we will see later, the answer he gives as to the distinguishing characteristic (the ceremonial law) is only partially correct, and in being such, it is the wrong answer to this most crucial question.
The "New Perspective on Paul"
In a similar vein, there are also a few Catholic apologists who have sided with the views of a new breed of Protestant exegetes. These Protestants have advanced what they call "The New Perspective on Paul." Current proponents of this new perspective are such Protestant names as James D. G. Dunn, E. P. Sanders, Alan Suggate, N. T. Wright and R. B. Hays, among others. Dunn's first attempt at advancing this theory came in the article "The New Perspective on Paul" (1983) and the book: The Justice of God: A Fresh Look at the Old Doctrine of Justification by Faith (1993), while E. P. Sanders wrote Paul and Palestinian Judaism (1977). N. T. Wright expressed his view in the article "Romans and the Theology of Paul" (1995); while Hays wrote "Three Dramatic Roles: The Law in Romans 3-4."
Although it is often touted as a "new" theory, in reality it stems from the views of Protestant William Werde in his 1897 German publication, which has since been translated into English under the title: The Task and Methods of New Testament Theology. The theory was also advocated by Protestant Kristar Stendahl in his 1970 work The Apostle Paul and the Introspective Conscience of the West, and now reprinted under the title: Paul Among the Jews and Gentiles.
The theory claims that the traditional interpretation which views St. Paul's writings as portraying a contest between "grace versus works" is not the main issue, and perhaps not even correct. Dunn hypothesizes that the emphasis on "grace versus works" is merely a product of the polemics circulating during the Reformation period between Luther and the Catholic Church. Rather, Dunn, et al, say that the Jew of the first century AD believed he was within the grace of God, and thus a struggle between grace and works was not the Jew's concern.
Consequently, Dunn claims that the central issue concerning the Justification controversy portrayed by the New Testament is sociological rather than soteriological; a matter of "Jew versus Gentile," not "grace versus works." The main challenge for the Jew is said not to be one of relinquishing his dependence on works and resigning himself to God's grace, but of accepting Gentiles as part of the covenant community, and letting them share in the graces of God that the Jew already has. In short, Dunn's thesis is that the Jew was not so much proud of his works as he was proud of his grace; whereas the traditional view says that, except for a remnant, the Jew was eliminated from the grace of God due to his obstinate reliance on works.
Following Kristar Stendahl, other Protestants such as Lloyd Gaston (Paul and the Torah, 1987) and Stanley Stowers (A Rereading of Romans: Justice, Jews and Gentiles, 1994) have taken the theory so far as to say that Jews and Gentiles have "separate but related ways" to salvation, with Israel continuing to live by the law as its accompaniment to salvation.
Dunn also seeks to apply his interpretation to our current day, teaching that mankind's real challenge is that he must learn to accept everyone regardless of race or ethnic background, since we are all God's children. Dunn and Suggate also postulate, for example, that Adolph Hitler's main problem was not one of intrinsic evil or hatred against God but merely a superiority attitude against people of other ethnic backgrounds.
On the surface, some parts of Dunn's view may sound logical, at least to some extent. The big question is, however: does the New Testament portray the Jews in the way Dunn suggests? Quite simply, the answer is NO; the New Testament does not focus on the ethnic paradigm Dunn is suggesting. If the New Testament hints at it in any way, it is only as a tangent to the bigger story of man's individual responsibility to reject his own self-righteousness and self-reliance so that he can receive the grace of God, which is his only means of salvation.
For a qualified Catholic critique of Dunn's view, see Brendan Byrne, S.J., The Problem of NovmoV and the Relationship with Judaism in Romans. Among other things, Byrne points out that "works of the law" refers to "the Jewish law in toto"; and that the famous Qumran document 4QMMT, which has been touted by followers of Dunn, turns out to be a Hebrew phrase meaning simply "some precepts of the Law" without the connotation of performance (See M. Bachmann's "4QMMT und Galaterbrief, und hrwth yv[m ERGA NOMOU" in ZNW 89  91-113).
Paul is clear, for example, in Romans 9:31-32 that, regardless of how the Jews may have thought of themselves as being in God's graces, the fact is that Scripture portrays them as pursuing righteousness by works, not of being overly proud of grace. Paul writes: 31 "...but Israel, pursuing a law of righteousness, did not arrive at that law. 32 Why? Because they did not pursue it by faith, but as though it were by works." It is only the "remnant" of Jews that remained in the grace of God, and the rest in Israel were hardened in their sin; and according to Paul, this occurred in the eighth century BC before the Gentiles ever became a concern (Romans 11:1-10). We will say more about this in the analysis below.
"Works of the Law"
As for the argument that the "works of the law" applies to the ceremonial law, such that Paul is teaching that the ceremonial law cannot justify but that the moral law does justify, the first thing I would like to mention is that the Council of Trent, which is our central authoritative source on matters of Justification, NEVER used such argumentation. (Nor did they use anything close to Dunn's view, noted above). This fact becomes significant for our investigation, since during the Counter-Reformation there were certain Catholic clerics who, in opposition to the Lutherans, were trying to advance the argument that "works of the law" referred only to the ceremonial law. As it stands, the Council of Trent rejected that apologetic.
In the sixth session of the Council (where Justification is addressed), neither the words "ceremonial law," "ritual practices," nor anything of the sort are mentioned, not even one time. The only time the Council mentions the word "circumcision" is in Chapter 7 when it is quoting from Galatians 5:6 ("in Christ Jesus neither circumcision avails anything, nor uncircumcision, but faith, which worketh by charity"), but it gives no elaboration on the usage of the term. Again, this is significant because it shows us that the Council did not think the "works of the law = ceremonial law" argument was a good, or even biblical, argument to explain the nature of Justification.
Rather than focus on the ceremonial law, the Council of Trent went right to the main, overarching issue, that is, the issue concerning "grace versus works" that I mentioned above. In the very first Canon the Council says:
"If anyone shall say that man can be justified before God by his own works which are done either by his own natural powers, or through the teaching of the Law, and without divine gracethrough Christ Jesus: let him be anathema."
Notice that the Council's view of "works" includes ANY kind of work, whether the work stems from one's "own natural powers" or "through the teaching of the Law." In the Council's mind there is no distinction between "ceremonial" works and "moral" works, at least in regard to how a man is justified before God.
Thus, the Council's tactic is to make an immediate antithesis between "works" and "grace." In the remaining 32 Canons, the Council continues the same argument, never once trying to settle the issue by an appeal to the ceremonial law of Israel, or an antithesis between Jew and Gentile.
The Council twice mentions the "Jews," but in neither case does it make a dictinction between the ceremonial law and the moral law of the Jews. The two references are in Chapter 1 and 2 of the Sixth Session: (1. "not even the Jews by the very letter of the law of Moses were able to be liberated (from the power of the devil and of death"; 2. "that He might both redeem the Jews, who were under the Law").
Again, in Chapter 8, Trent states: "...and are, therefore, said to be justified gratuitously, because none of those things which precede justification, whether faith, or works, merit the grace itself of justification; for, ‘if it is a grace, it is not now by reason of works; otherwise (as the same Apostle says) grace is no more grace' [Romans 11:6]." Obviously, if Trent includes "faith" as "none of those things" which can justify, then surely moral works are included in the "none."
Now one might argue that by these injunctions Trent was merely denying Pelagianism and semi-Pelagianism. First, Trent never makes such a claim. In fact, the very foe they were fighting, Martin Luther, was the one accusing the Catholic Church of Pelagianism. Second, if Trent used some other kind of argumentation other than the one presented in Canon 1 and Chapter 8, namely, an argument that focused on the ceremonial law as the exclusive meaning of the "works of law," then the objection could be sustained. But such is not the case.
The point remains that Trent NEVER sought to answer the question of Justification by dissecting the Law into its constituent parts, i.e., ceremonial, moral or civil precepts. Although they had every opportunity to do so, the Council simply did not cite any verses from the New Testament that single out the ceremonial law. They only quoted from the NT passages which view the Law in its totality, since their main objective was to distinguish grace from law, not grace from ceremonies.
Logic dictates that if the ceremonial law apologetic was so crucial to the understanding of the issue of Justification (as some modern Catholic theologians claim) then Trent would have been REQUIRED to use it. They would have no right to ignore it in favor of a view which taught that the Law referred to the WHOLE law of Moses and Works referred to ANY work.
Now some might argue that the Council's focus was dictated by the particular arguments that the Reformers were advancing; and since this is not our concern today, nor was it the concern of Paul in the first century AD, then we are not obligated to use it. Let me say quite candidly, this is wrong.
First, as I noted above, the Council of Trent already ignored the "ceremonial law" argumentation which was being advanced by various Catholic clerics who were trying to answer the Lutherans.
Second, and this should come as no surprise to Catholics who know their history, the Fathers of the Church show quite clearly in their writings that there was a consensus of understanding that, in reference to how a man is justified, the words "works of the law," "works," or "law" referred to ANY work, ceremonial or moral."