Against You, You only, have I sinned and done what is evil in Your sight. . . . Create in me a clean heart, O God, and renew a right spirit within me.
PSALM 51:4, 10
The Holy Spirit speaks the same way in Psalm 32:5: "I said, I will confess my transgression to the Lord; then Thou didst forgive the guilt of my sin." For the forgiveness of sins, therefore, this confession or knowledge is necessary, that we believe and confess that we are sinners and that the whole world is under the wrath of God. Thus the First Commandment denounces sin by its very promise. God promises: "I am the Lord, your God; that is, I am He through whom salvation will come to you against death and sin." This itself argues that our whole nature is punishable by death and sin. Why else should He promise that He will be God to us? Thus the Word of God—that is, both the Law and the Gospel, or promise—proves with clear and certain arguments that we are sinners and are saved by grace alone. If God promises life, it follows that we are under death. If He promises forgiveness of sins, it follows that sins dominate and possess us. Now, the wages of sin is death (Romans 6:23). Both the threats and the promises all show the same thing. . . . The divine voice and the promise of salvation were addressed to us men, against death, sin, and hell.
From Commentary on Psalm 51 (Luther's Works 12:340)