"As long as a person is not a murderer, adulterer, thief, he would swear that he is righteous. How is God going to humble such a person except by the Law?...
As long as a person thinks he is right he is going to be incomprehensibly proud and presumptuous. He is going to hate God, despise His grace and mercy, and ignore the promises in Christ. The Gospel of the free forgiveness of sins through Christ will never appeal to the self-righteous. This monster of self-righteousness, this stiff-necked beast, needs a big axe. And that is what the Law is, a big axe. Accordingly, the proper use and function of the Law is to threaten until the conscience is scared stiff....
The Law is a mirror to show a person what he is like, a sinner who is guilty of death, and worthy of everlasting punishment. What is this bruising and beating by the hand of the Law to accomplish? This, that we may find the way to grace. The Law is an usher to lead the way to grace."
Martin Luther, Commentary on Galatians