From Luther’s Epistle Sermon for the Third Sunday after Easter (1 Peter 2:11–20):
We have heard above that the two parts are to be together in a Christian and emphasized in Christan teaching. The first part is faith, that we are redeemed from sin through the blood of Christ and have forgiveness. The second part, after we have [faith], is that afterward we should become different people and live a new life. In Baptism, or when we begin to believe, we receive not only the forgiveness of sins (which is the grace that makes us God’s children) but also the gift that must do away with the remaining sins and kill them. Our sins are not forgiven so that we would continue in them (as St. Paul says in Romans 6), as the insolent spirits and despisers of grace allege. Rather, even though sins have been blotted out through Christ’s blood, so that we do not need to pay or make amends for them, and we now are children of grace and have forgiveness, yet that does not mean sin has been entirely done away with and killed in us.
The forgiveness of sins and the killing of them are two different things. Both of them must be proclaimed against those who confuse and turn things upside down with false doctrine. Against the first, the pope and many others have taught that the forgiveness of sins is to be obtained through the trickery of their own self-chosen and invented works and their own satisfactions. This error always continues in the world from Cain at the beginning to the end. Then, when this error has been put down, there are again false spirits on the other isde, who have heard the preaching about grace and boast about it and yet produce nothing more from it, just as if that were enough, and forgiveness should do nothing more in us than that we remain as we were before. Afterward, there were just as many as before, when we still knew nothing at all about Christ and the Gospel.
Therefore, those who want to be Christians must know and learn that, since they have obtained forgiveness without their own merit, they must from now on not allow or indulge in sin, but rather oppose their former, evil, sinful lusts and avoid and flee their work and fruits. That is the summary and meaning of this Epistle reading.
Luther’s Works, Vol. 78: Church Postil III (St. Louis: CPH, 2014), 154–155