It's often struck me how...slippery some of the saved-by-works folk must become in countering the plain declaration of God's word that by the works of the law no one is justified before God. "It's only the Mosaic Law that Paul's talking about," they say. "There are other commands of God outside the Mosaic Law that MUST be obeyed if you want to be saved." However, in Scripture, salvation by grace through faith in Christ is set in contradistinction to salvation by ritual, self-effort and rule-keeping. Law-keeping is representative of a way of interacting with God that doesn't "work," and that is actually the opposite of the way God has made through Christ to enjoy fellowship with Himself.
The Law (Mosaic, or otherwise) is the Old Covenant means to pleasing and serving God, to be in His "good graces." Obey the rules of separation, and ceremony, and conduct that God had issued through Moses and you'd be all right with Him. But, as Scripture tells us, this was an impossible way to maintain good relations with God. Over and over again, Israel wandered from God into terrible sin. Keeping God's law successfully - which is to say, perfectly - God's own Chosen People had demonstrated for centuries was impossible. Read Psalm 78. As Paul explained, the Law that was good, served only to condemn rather than enable people to walk with God.
Romans 7:5-12
5 For while we were in the flesh, the sinful passions, which were aroused by the Law, were at work in the members of our body to bear fruit for death.
6 But now we have been released from the Law, having died to that by which we were bound, so that we serve in newness of the Spirit and not in oldness of the letter.
7 What shall we say then? Is the Law sin? May it never be! On the contrary, I would not have come to know sin except through the Law; for I would not have known about coveting if the Law had not said, "YOU SHALL NOT COVET."
8 But sin, taking opportunity through the commandment, produced in me coveting of every kind; for apart from the Law sin is dead.
9 I was once alive apart from the Law; but when the commandment came, sin became alive and I died;
10 and this commandment, which was to result in life, proved to result in death for me;
11 for sin, taking an opportunity through the commandment, deceived me and through it killed me.
12 So then, the Law is holy, and the commandment is holy and righteous and good.
God's commands serve only to provoke Man's sinful inclinations. Holy, righteous and good as God's laws are, the sinful passions of human beings are aroused by His laws, not subdued by them. And "falling foul" of God's law brings us all under His condemnation, not into fellowship with Him. Here, though, it's important to take in what Paul is actually saying. All human beings, inside or outside of Israel, Jew or Gentile, are under divine condemnation. But if the Mosaic Law, given solely to Israel, is what Paul had in mind when he wrote of "the Law," how does it apply to Gentiles? How do they fall condemned under a system of law God never intended for them? Clearly, Paul had more in mind when he wrote of God's law than the laws of separation, ceremony and civil conduct given to Israel through Moses. Paul's notion of "the Law" encompassed commands of God beyond those given to Israel, commands that were strictly confined to the Mosaic system of laws. This is very evident in his letter to the Galatian believers who were mostly Gentiles.
Galatians 3:10-14
10 For as many as are of the works of the Law are under a curse; for it is written, "CURSED IS EVERYONE WHO DOES NOT ABIDE BY ALL THINGS WRITTEN IN THE BOOK OF THE LAW, TO PERFORM THEM."
11 Now that no one is justified by the Law before God is evident; for, "THE RIGHTEOUS MAN SHALL LIVE BY FAITH."
12 However, the Law is not of faith; on the contrary, "HE WHO PRACTICES THEM SHALL LIVE BY THEM."
13 Christ redeemed us from the curse of the Law, having become a curse for us—for it is written, "CURSED IS EVERYONE WHO HANGS ON A TREE"—
14 in order that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we would receive the promise of the Spirit through faith.
Why is no one - Jew or Gentile - justified by keeping God's commands (vs. 11)? Because we don't have it in ourselves to obey perfectly, as we must if rule-keeping is going to bring us into favorable relations with God. Paul explained this very well in Romans 7 (see above). God is perfect and accepts only perfection in our interactions with Him. But we are none of us perfect. Far from it (Romans 3:10, 23; Ephesians 2:1-3; Titus 3, etc.). We need another way to be reconciled to, and accepted by, our holy Maker, a way to be perfect that doesn't originate in us.
Galatians 3:21-25
21 Is the Law then contrary to the promises of God? May it never be! For if a law had been given which was able to impart life, then righteousness would indeed have been based on law.
22 But the Scripture has shut up everyone under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
23 But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed.
24 Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith.
25 But now that faith has come, we are no longer under a tutor.
Here, Paul indicated that keeping of the rules cannot "impart life." We are all "dead in trespasses and sins" and desperately need reviving, but the Law (representative of a particular fleshly, legalistic way of approaching God) is unable to impart to us the perfect life of Christ, which is what we need. Law-keeping can only condemn imperfect creatures. Instead, God has instituted righteousness by faith; He will justify us by way of our trust in Christ by which his perfect righteousness is then imputed to us. (1 Corinthians 1:2, 8-9, 30; Ephesians 1:1-13) Justified by faith, we are no longer under the "tutor," the death-dealing commands of God.
Continued below.