Luther on the 10 Commandments, Psalm 51:1

[GHansen]

Ps. 51:1  Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions .

But after we have said how God the Righteous and man the sinner are to be reconciled, we must also remember to look properly at the word “Have mercy.” If we considered it more carefully, we should have to declare that our whole life is enclosed and established in the bosom of the mercy of God. Since we are all the “me” here, that is, sinners, the conclusion follows clearly and necessarily that whatever we are and live is all by sheer grace, not by our righteousness or merit. “What then,” you say, “ought not the Ten Commandments to be kept? And if they are kept, is not that righteousness?” I answer: We want to keep and observe the Ten Commandments, but with a large, that is, truly evangelical dispensation or distinction. We have received only the first fruits of the Spirit (Rom. 8:23), and the sighs of the Spirit remain in our hearts. Our flesh with its lusts and desires remains, too, the whole tree with its fruits. This is the reason why the Ten Commandments can never be kept fully. Otherwise, if the Ten Commandments could be kept in their entirety, what need would there be of the righteousness for which David prays in the word “Have mercy”? What need would there be of imputation? Now, since even in the saints there are still remnants of sin that have not yet been completely mortified, two things happen: Through the Spirit dwelling in us we resist sin and obey the Ten Commandments; and yet, driven to sin by flesh and Satan, we hope for the forgiveness of sins.

Thus sacrifice was a form of obedience under the Law, and yet the prophet says (v. 16): “Thou hast not wanted sacrifice and burnt offering.” They were sacrifices, but in the sense that they did not do away with mercy. In the same way we keep the Law through the Holy Spirit, and yet the word “Have mercy” remains; that is, we remain sinners and have need of the free forgiveness of sins through the merit of Christ. Thus mercy is our whole life even until death; yet Christians yield obedience to the Law, but imperfect obedience because of the sin dwelling in us. For this reason let us learn to extend the word “Have mercy” not only to our actual sins but to all the blessings of God as well: that we are righteous by the merit of another; that we have God as our Father; that God the Father loves sinners who feel their sins—in short, that all our life is by mercy because all our life is sin and cannot be set against the judgment and wrath of God.

Luther, M. (1999). Luther’s works, vol. 12: Selected Psalms I. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.) (Vol. 12, pp. 320–321). Saint Louis: Concordia Publishing House.