Now we know that what things that the law says, it says to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. 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. (Rom. 3:19-20)

The purpose of the law was never to give man a standard he must keep. The purpose was to reveal the righteousness of God in written form, and to prove to man that he could never be righteous enough for God. In short, the law exposes what we ARE. Stand face to face with the holy, just, and good law of God, and you will be exposed as unholy, unjust, and bad.
If a person is honest, the law will stop their mouth – they will stop talking about the possibilities of their own righteousness through works. The more we try to keep any law of God, the more that law will probe and expose us as hopeless. But that is not the end of the purpose of the law: It is to bring us to where we will fall to our knees and put our faith in the Person of Jesus Christ – and then live in God’s grace and Truth.