Sin is sin, and by its nature it merits punishment, whether you acknowledge it or not. The acknowledgment of sin is a sort of co-requisite [for justification] in that God wishes to forgive sins to those who acknowledge them and does not wish to forgive sins to those who do not acknowledge them. This promise is the sole cause, the first, middle, and last cause; that is, it is everything in justification. At this promise David looks when he says, “For I know my iniquity.” It is as though he would say: “I am not citing as some sort of merit the fact that I acknowledge my sin. But because Thou hast promised grace to those who acknowledge their sin, therefore I come before Thee to acknowledge my sin.”
Martin Luther commenting on Psalm 51:3
Luther’s Works v. 12 333