What "Law" is Paul referring to in Romans 3:20? According to the immediate context, which begins in verse 9 and extends through verse 18, it is the Law found in the Psalms which convicts both Jew and Gentile in sin. This global conviction in sin is evident as Paul says in verse 9 that "both Jews and Gentiles are all under sin," and then proceeds to quote from seven different Psalms which show that the whole world is condemned in sin because of the Law found in the Psalms (5:9; 14:1-3; 10:7; 36:1; 53:1-4; 59:7; 140:3).
It is obvious, then, that the "Law" or "works of the law" cannot refer only to Jewish ceremonies, for the Psalms Paul cites do not refer to ceremonies but to the general laws of God which convict all men in sin. This conviction of sin is precisely why Paul closes in Romans 3:20 with the statement "...for through the Law comes the knowledge of sin." Conversely, there is no passage in the Bible which says that the ceremonial law convicts men in sin, let alone say that the ceremonial law could convict a Gentile in sin. No, the Law of Romans 3 refers to the WHOLE system of law and it is that Law which convicts men in sin.
Paul gives the same truth in Romans 5:20:
"The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more."
As in Romans 3:20, here the Law serves the purpose of exposing sin. What Law is Paul referring to? According to the context it refers to the Law that came with Moses, as Paul says in verses 13-14: "for until the Law sin was in the world, but sin is not imputed when there is no law. 14 Nevertheless death reigned from Adam until Moses...," but Paul says nothing about this Law referring only to the ceremonial law. The context is clear that the Law refers to the whole law of Moses.
We see the same truth in Romans 2:17-23.
17 But if you bear the name "Jew " and rely upon the Law and boast in God, 18 and know His will and approve the things that are essential, being instructed out of the Law, 19 and are confident that you yourself are a guide to the blind, a light to those who are in darkness, 20 a corrector of the foolish, a teacher of the immature, having in the Law the embodiment of knowledge and of the truth, 21 you, therefore, who teach another, do you not teach yourself? You who preach that one shall not steal, do you steal? 22 You who say that one should not commit adultery, do you commit adultery? You who abhor idols, do you rob temples? 23 You who boast in the Law, through your breaking the Law, do you dishonor God?
Here Paul says that the Jews "rely on the Law and boast in God" (verse 17) and then in verses 21-22 Paul specifies that Law as the commands against "stealing" (verse 21); "adultery" (verse 22); and "idol worship" (verse 22). These three commands come from the Ten Commandments. "Stealing" and "adultery" represent the sixth and seventh commandment, while "idol worship" represents the first commandment.
We can see the same truth in Romans 4:3-4:
3 For what does the Scripture say? "Abraham believed God, and it was credited to him as righteousness." 4 Now to the one who works, his wage is not credited as a gift, but as what is due.
This is the controlling verse for much of what Paul says in Romans. These verses tell us that the principle issue at stake is one of "gift versus debt," or, as we have seen, "grace versus works." This is the fundamental principle in Paul's teaching, and those who miss it go off into many other irrelevant tangents.
Why can't "works of the law" justify a man? Is it because Jewish ceremonies are now obsolete and faith came to replace them? No, Jewish ceremonies NEVER saved any Jew. The real reason "works of the law" can't justify is that they are in a legal system devoid of grace. They come from the Old Covenant, a covenant which was based on law.
The grace of the Old Covenant had been squeezed out when Adam sinned against God. From then on, according to Paul in Romans 4:3-4, if one tries to base his justification on law, it would be just as if he were trying to put God into debt to pay him a wage. Paying wages to those who work is representative of a system devoid of grace. But Justification cannot be bought, since God owes no man anything. Therefore, justification cannot be given on a wage basis; rather, it must come by grace, initiated by faith in God. It was the New Covenant that brought grace back into the picture, and when it came it set aside the Old Covenant with its legal system of moral, ceremonial and civil laws.
"Works of the Law" - Page 8