There is only one cause for justification, namely, the merit of Christ, or the gracious mercy which hearts that are ignited by the Holy Spirit grasp by faith. If someone wants to, he may list the acknowledgment of sin as a second cause … It is the sort of cause that the whole thing still depends on the mercy of God or on the promise, that God has promised He will have mercy on those who acknowledge their sins and thirst for righteousness (Matt. 5:6). Otherwise, if you are talking about the nature of sin, even about the “conscious sinner” … on the basis of Law and nature, he deserves nothing but the deepest punishment and wrath. When such people escape punishment and wrath, it is all by the mercy of God, who has promised that by freely forgiving their sin He wants to give life to those who feel their sins and the horrors of divine judgment. There is nothing that could in any way be cited as merit, because even the acknowledgment of sin is nothing except what the divine promise makes it.
Martin Luther commenting on Psalm 51:3
Luther’s Works v. 12 332