Could the law help a Jew keep the law?
No, the law could not help a Jew keep the law. It had no power, even for the Jew, to help him be a law keeper; all it could do was condemn. Paul wrote that by the deeds of the law no flesh is justified, for the law gives the knowledge of sin.
Les explains that the law could do nothing but condemn. It had no power, even to the Jew, to help the Jew be a law keeper. Its purpose was to point the finger, declare guilt, and provide the knowledge or awareness of sin—what Les identifies as old Adam. But then comes the contrast: the righteousness of God without the law. This is not human righteousness, which is like filthy rags, but God's righteousness. Les also reminds us of two fixed precepts: without faith you cannot please God, and without the shedding of blood there is no remission, which points to the cross.
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.