We've often talked about the three functions or uses of the law, Curb, Mirror and Guide. I suppose this discussion would fall under function number 2, the Mirror. In terms of how the law shows us our sin, or, what we see in the mirror when we look. There are three ways of looking.
1) Self-righteous hypocrisy
Some hear the law and say, "I have kept this", much like the rich young man (Mark 10) who questioned Jesus. But wanting to justify himself, he couldn't see that he actually broke the law. The law, to him, was a weak shadow of the true law. This was not because of a lack in the preacher (Jesus), but the hardness of his heart and the rationalization of his mind kept him from hearing the law's perfect demands.
To be sure, some preachers lend this kind of law all on their own, however. A de-fanged, de-clawed law that doesn't kill but only roughs you up a bit before you dust yourself off and feel even more righteous. This reaction to the law is poisonous to faith, because it obscures our need for a savior, it covers our true depravity with a fig leaf of supposed good works, and it leaves the sinner in self-deception that he is right with God on his own merits.
2) Despair and unbelief
Truly sad is the sinner who sees his sin, and perhaps even feels sorry for it, but sees no hope of remedy. If we look only within ourselves, it's true, we are doomed. We can't hope to repay God for our misdeeds. We can't hope to straighten up from here on out. We are blind, dead, and at war with our very creator. Truly seeing behind the veneer of a self-righteous hypocrisy might lead one to utter despair. The conscience bears down on you like a boulder on your heart, such a worm, so despicable.
The hopelessness of this despair reminds us of Judas, who felt sorry for his sin, but having no faith or hope in Christ, hanged himself. Despair and unbelief are worse - a kind of spiritual suicide.
3) Contrition which seeks Christ for forgiveness
The great blessing of the law though, is that it drives us to despair - but in preparation for the hope and joy of the Gospel! To die, only to know the life he brings. Yes, without knowing our sin, how could we know our savior? Without knowing the severity of our sin, how could we appreciate the depth of his forgiveness? Without a daily, hard, cold look in the mirror, and a true view of the ugliness of our own sin, how can we daily know the forgiveness that flows from our baptism? This is the great blessing of the law - that it prepares us for the Gospel. It is the diagnosis before the treatment, the plowing of the dead field before the seed is planted and flourishes. The law lowers us down into the grave and shovels on the dirt, only for the trumpet call of Christ to bust our tombs open and burst death open into life.
Here our biblical example is King David, who, called out for his sin with Bathsheba - confessed it and looked to God for mercy. The words of Psalm 51, "create in me a clean heart, oh God, and renew a right spirit within me" are the words of a penitent but faithful man who looks to a merciful God for forgiveness. In other words, a Christian.
Thanks be to God for the Law, holy and perfect, which shows us our sin, wretched and vile as it is. And thanks be to God for the Gospel of Jesus Christ, whose holy and perfect life and death make us holy and perfect by grace through faith in him.