Why can't the law justify anyone?
Because the law's demands — "thou shalt, thou shalt not" — apply to every mouth, Jew and Gentile alike, so all the world becomes guilty before God. By the deeds of the law no flesh is justified in His sight; the law's function is to give the knowledge of sin, not justification.
Les explains that the whole system of law was given to the nation of Israel, and they alone were under it. But it doesn't stop there: the indictment is not just against Jewish mouths but against every mouth, which includes everybody — Jew and Gentile, black and white, rich and poor, regardless of a person's status. Everyone is under that demand of the law, so all the world becomes guilty before God. Most people think that if they can just keep the law they'll make it, but the word is not "righteous," it is "guilty." Therefore, since the law — the Mosaic Ten Commandments — makes everyone guilty, by the deeds or the keeping of the law shall no flesh be justified in his sight. As Les puts it, "for by the law is the knowledge of sin," and he urges his listeners to underline that word sin.
Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
More Questions
Why can't a person be justified by the deeds of the law?
Doing the law's deeds cannot justify anyone in God's sight because the law gives knowledge of sin, not knowledge of salvation or a way to heaven.
Can a person be justified by doing what the law requires?
No flesh is justified before God by the deeds of the law; the law only brings the knowledge of sin.