The following argument is excerpted, with some modifications, from two sources: The appendix to my essay Why Justification Still Matters, and from my essay De Iustificatione Dei, which compares the soteriologies of Augustine and Martin Luther. highly recommend that those interested in this debate read those two essays in full after having read this excerpt for more information on the Justification debate.
In response to the common Protestant use of Romans 3:28 to prove the doctrine of Justification by Faith Alone, Roman Catholic apologists often argue that when Paul talks about “works of the Law”, he is only excluding ceremonial works, not moral works. For example, Roman Catholic apologist Tim Staples argues that “works of the Law,” is nothing more than a reference to circumcision:
One of the problems in Rome St. Paul was dealing with was a very prominent heresy known to us today as the “Judaizer” heresy. Those attached to this sect taught belief in Christ and obedience to the New Covenant was not enough to be saved. One had to keep the Law of Moses, especially circumcision, in order to merit heaven.
Staples then goes on to argue that since Paul’s definition of “works of the Law” does not include the Decalogue, obedience to the Decalogue may still be regarded as a means by which the saving grace of God comes upon individuals:
We are bound to follow “the law of Christ” as St. Paul said in I Cor. 9:21, but we must understand that we are saved by grace through the instruments of faith and obedience. That obedience includes keeping the Ten Commandments, but the keeping of the commandments is an instrument—a necessary instrument—through which the grace of God flows and keeps us in Christ, the principle of reward for us. Thus, we have to keep the commandments to be saved, but we understand it is only through grace that we can do so.
Furthermore, he argues that while the initial grace of Justification is by faith alone, once one has received that initial grace, it is up to our works to keep us in that state of Justification:
St. Paul is in no way eliminating works in any sense, to be necessary for salvation; he is simply pointing out what the Catholic Church has taught for 2,000 years: there is nothing anyone can do before they enter into Christ that can justify them. But once a person enters into Christ… it’s a whole new ballgame (see Phil. 4:13; Rom. 2:6-7; Gal. 6:7-9, etc.).
Aside from the fact that this argument contradicts the supposed “consensus” between Lutherans and Roman Catholics in the Joint Declaration on the Doctrine of Justification, that “whatever in the justified precedes or follows the free gift of faith is neither the basis of justification nor merits it” (par. 25), it runs into all sorts of exegetical problems once the larger Pauline context is taken into consideration. It is hard to maintain that “works of the law” does not apply also to the Decalogue when, for example, Paul mentions the command not to covet as one of those works (Romans 7:7). Also, Staples’ argument was long ago refuted by Martin Luther in the Heidelberg Disputation, who borrows from Augustine’s exegesis of Romans in De Spiritu et Litera to prove that Paul meant to exclude all works from his view of justification, not just ceremonial works:
This is made clear by the Apostle in his letter to the Romans 3[:21]: “But now, apart from law, the righteousness of God has been disclosed,” St. Augustine explains this in his book, On the Spirit and the Letter: “Without the law, that is, without its support.” In Rom. 5[:20] the Apostle states, “the law came in, with the result that the trespass multiplied,” and in Rom. 7[:9] he adds, “but when the commandment came, sin revived.” For this reason, he calls the law a “law of death” and “a law of sin” in Rom. 8[:2]. Indeed, in 2 Cor. 3[:6] he says, “the letter kills,” which St. Augustine throughout his book, The Spirit and the Letter, understands as applying to every law, even the holiest law of God.
Note that Luther is not inventing this interpretation out of thin air, but is drawing from the biblical exegesis of Augustine of Hippo. The view championed by Augustine is that “works of the Law” refers to the entire Mosaic Law, including the Decalogue. Augustine argues this in The Spirit and the Letter, wherein the whole premise of the treatise is that the entire Law is incapable of providing salvation. He writes in chapter 21:
The law, then, of deeds, that is the law of works, whereby this boasting is not excluded, and the law of faith, by which it is excluded, differ from each other; and this difference it is worth our while to consider, if so be we are able to observe and discern it. Hastily, indeed, one might say that the law of works lay in Judaism, and the law of faith in Christianity; forasmuch as circumcision and the other works prescribed by the law are just those which the Christian system no longer retains. But there is a fallacy in this distinction, the greatness of which I have for some time been endeavoring to expose . . .
Further down in the same chapter, Augustine cites Romans 7:7-12. He notes that in verse 7, Paul refers to the tenth commandment. From this, he infers that “works of the law” must encompass the Decalogue as well:
Now, the apostle says that that law by which no man is justified, entered in that the offence might abound, and yet in order to save it from the aspersions of the ignorant and the accusations of the impious, he defends this very law in such words as these: “What shall we say then? Is, the law sin? God forbid. Nay, I had not known sin but by the law: for I had not known concupiscence, except the law had said, Thou shalt not covet. But sin, taking occasion, wrought, by the commandment, in me all manner of concupiscence,” He says also: “The law indeed is holy, and the commandment is holy, and just, and good; but sin, that it might appear sin, worked death in me by that which is good.” It is therefore the very letter that kills which says, “Thou shalt not covet,” and it is of this that he speaks in a passage which I have before referred to: “By the law is the knowledge of sin…”
He then concludes that these works are excluded as pre-requisites for justification:
Now, having duly considered and weighed all these circumstances and testimonies, we conclude that a man is not justified by the precepts of a holy life, but by faith in Jesus Christ,—in a word, not by the law of works, but by the law of faith; not by the letter, but by the spirit; not by the merits of deeds, but by free grace.
This is the book by Augustine that Luther cites in his proof for the first thesis of his Heidelberg Disputation, as cited above. This shows the direct continuity between Augustine and Luther in their reading of Paul.
Furthermore, Augustine’s interpretation continued into the Middle Ages in the writings of Thomas Aquinas. David S. Yeago, citing Augustine and Aquinas, makes this point. He states:
I stand by the Augustinian arguments for taking “works of the law” in Paul as covering the whole law, not just the ceremonial “identity-markers” that distinguished Jews from Gentiles. As Aquinas points out, it is a bit hard to see how “knowledge of sin” would come from the commandments to circumcise male infants and keeping kashrut. “And so the Apostle means that a human being is not justified by any works of the law, even those mandated by the moral precepts…” Aquinas, Super Epistoulas I., ad loc., 297.
In conclusion, the Roman Catholic argument as presented by Tim Staples does not hold water. To argue that Paul is merely excluding ceremonial works and not all works when he speaks of “works of the law” does not stand up to scrutiny when examined in the larger context of Romans. Furthermore, the interpretation that Paul is excluding all works from Justification is not a novel idea, but has a long history going back to Augustine.
- Why Justification Still Matters
- De Iustificatione Dei: Luther as Interpreter of Augustine’s Soteriology
 Tim Staples, “Are Good Works Necessary for Salvation?,” Catholic Answers, Last Modified April 30, 2015, https://www.catholic.com/magazine/online-edition/are-good-works-necessary-for-salvation.
 Herein lies the inherent contradiction in Catholic Answers’ apologetics against Protestantism: Their authors claim that a consensus exists on both sides of the Tiber on the key issues pertaining to Justification, but at the same time, they argue against the very same points which they claim that we have a consensus on!
 Hans J. Hillerbrand, Kirsi I. Stjerna, and Timothy J. Wengert (eds.), The Annotated Luther, Vol. 1: The Roots of Reform (Minneapolis, MN: Fortress Press, 2015), 88. The full text of the Heidelberg Disputation is available online at http://bookofconcord.org/heidelberg.php.
 Augustine of Hippo, “On the Spirit and the Letter,” 13.21, in Saint Augustin: Anti-Pelagian Writings, ed. Philip Schaff, trans. Peter Holmes, vol. 5, Nicene and Post-Nicene Fathers (New York: Christian Literature Company, 1887).
 Ibid., 13.22.
 David S. Yeago, The Apostolic Faith: A Catholic and Evangelical Introduction to Christian Theology, vol. 2. (n.p., n.d.), 177 n.36.