Dr. Martyn Lloyd Jones – (Romans Exposition, Chapters 3:20-4:24, pp 21-22):

The whole function of the Law is to define sin, to reveal its nature; and that is why we are without any excuse at all. The law is in our hearts; but that is not clear enough, so God made it explicit. He had defined it, He has underlined it, He has shown it plainly in the written Law give to the Jews.

. . . The Law was given to pinpoint sin, to define it, to bring it out of its hiding-place and to show its exceeding sinful character. . . Nothing so shows the exceeding sinfulness of sin as the Law itself does; and once a man has seen the real meaning of the Law he sees the foulness, the vileness of his own nature.

. . . The Law was never given to save man, but it was given as a “school-master” to bring him to the Savior. The whole object and purpose of the Law is to show man that he can never save himself. Once he has understood the Law and its spiritual meaning and content he knows that he cannot keep it. He is undone. . . What is the summary of the Law? It is: “Thou shalt love the Lord thy God with all thy heart, and all thy soul, and all thy mind, and all thy strength; and thou shalt love thy neighbor as thyself.” Has anyone done that and so kept the Law? No, “All have sinned and come short of the glory of God.” That is what the Law says. It shows us our utter helplessness and hopelessness, and thereby it becomes “our schoolmaster to lead us to Christ,” the only One Who by the grace of God can save us, and deliver us, and reconcile us to God, and make us safe for all eternity. Paul glories in the Gospel which proclaims that “the just shall live by faith,” because “by the deeds of the law shall no flesh be justified in His sight, because by the law is the knowledge of sin.”