I am reading Mere Christianity at the moment and was inspired to share this quote from C.S. Lewis on the purpose of the Moral Law in showing sinners their need for forgiveness:

Christianity tells people to repent and promises them forgiveness. It therefore has nothing (as far as I know) to say to people who do not know they have done anything to repent of and who do not feel that they need any forgiveness. It is after you have realized that there is a Moral Law, and a power behind the law, and that you have broken that law and put yourself wrong with that Power—it is after all this, and not a moment sooner that Christianity begins to talk.  

When you know you are sick, you will listen to the doctor. When you have realized that our position is nearly desperate you will begin to understand what the Christians are talking about. They offer an explanation of how we got into our present state of both hating goodness and loving it. They offer an explanation of how God can be this impersonal mind at the back of the Moral Law and yet also a Person. They tell you how the demands of this law, which you and I cannot meet, have been met on our behalf, how God Himself becomes a man to save man from the disapproval of God. It is an old story and if you want to go into it you will no doubt consult people who have more authority to talk about it than I have. All I am doing is to ask people to face the facts—to understand the questions, which Christianity claims to answer. And they are very terrifying facts. I wish it were possible to say something more agreeable. But I must say what I think true. Of course, I quite agree that the Christian religion is, in the long run, a thing of unspeakable comfort. But it does not begin in comfort; it begins in the dismay I have been describing, and it is no use at all trying to go on to that comfort without first going through that dismay.

—From Mere Christianity, Book One: Right and Wrong as a Clue to the Meaning of the Universe, Chapter 5: We Have Cause to be Uneasy