Romans 3:20

Therefore. — Rather, because. All mankind alike owe the penalty for their sins. Because not even the Law can protect its votaries. It has no power to justify. All it can do is to expose in its true colours the sinfulness of sin.

The proposition is thrown into a general form: not by the works of the (Jewish) Law, but by “works of law” — i.e., by any works done in obedience to any law. Law, in the abstract, as such, is unable to justify. It might perhaps, we gather from later portions of the Epistle, if men could really keep it, but no law can be kept strictly and entirely.

Knowledge of sin. — “Full and thorough knowledge.”

In the state anterior to law, man is not supposed to know what is sinful and what is not. Conscience, gradually developed, comes in to give him some insight into the distinction, but the full knowledge of right and wrong, in all its details, is reserved for the introduction of positive law. Law has, however, only this enlightening faculty; it holds the mirror up to guilt, but it cannot remove it.