What is the true purpose of the law?

The true purpose of the law is to convict people and give them the knowledge of sin. The law itself, the Ten Commandments, is holy, just, and good; it is not the problem. Rather, sin works through the law in human members to bring forth fruit unto death.

Les explains that old Adam kept Saul blind to the true purpose of the law, which was to convict him. The law does nothing but give the knowledge of sin, as Romans 3 states. The motions of sins which were by the law worked in the members to bring forth fruit unto death. Yet the law itself—the Ten Commandments—is holy, just, and good. It was not made death unto him; God forbid. The commandments are good; the working of sin through them is what brings death. Saul, the religious fanatic, was piling guilt upon guilt, headed for the great white throne, until the law’s convicting purpose finally reached him.

Romans 3:20

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.

What does the Law give us knowledge of?

The Law gives knowledge of sin. It was given not to help people do right, but to stop every mouth and show all the world guilty before God, revealing how far mankind falls short of God's standard, because no one can keep the Ten Commandments.