Charles Ellicott Commentary Romans 3:20

"because by the works of the law shall no flesh be justified in his sight; for through the law [cometh] the knowledge of sin." — Romans 3:20 (ASV)

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

The proposition is stated in 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 people could really keep it, but no law can be kept strictly and entirely.

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

In the state prior to law, people are not presumed to know what is sinful and what is not. Conscience, gradually developed, provides 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 capacity; it holds the mirror up to guilt, but it cannot remove it.