Therefore by the deeds of the law no flesh will be justified in his sight, for by the law is the knowledge of sin.
Romans 3:20.
The Jews were in possession of the scriptures of the Old Testament, among them God’s righteous demands in the law of Sinai. However, that did not make them any better than other nations; they did not become righteous thereby. And, indeed, it is not possible to become so before God by the “deeds of the law”. “For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all” (James 2:10). It was not the aim of God’s law to save sinful man, but rather to show him that he is guilty and needs salvation.
“By the law is the knowledge of sin.” Whoever is acquainted with God’s demands must realize that he does not, and indeed cannot, fulfil it completely and perfectly. The more someone endeavours, even sincerely, to keep God’s laws, the sooner he will be aware that he is governed by a sinful power that prompts him to transgress God’s righteous commandments.
Therefore the law proves to be “the strength of sin” (1 Corinthians 15:56), although it is holy and despite its commandments being holy and just and good (cf. Romans 7:12). So the law cannot bestow on anyone the righteousness to make him fit to stand before God. It cannot forgive his transgressions, nor free him from the power of the sin that indwells him.
Image: By Guido Reni – Web Gallery of Art: Image Info about artwork, Public Domain, https://commons.wikimedia.org/w/index.php?curid=15885392