In 1532 Martin Luther preached a sermon at the funeral of Duke John of Saxony. He preached on 1 Thessalonians 4:13-14. It’s a good sermon in many ways. One helpful part of this sermon is where Luther explained how Satan, the accuser, uses the law in a crafty way. He first tells us that we have to be good and keep the law, but then he reminds us that we haven’t kept the law. “And with that thought he brings one into such anxiety that one is ready to despair.” Luther continues:
And again when occasionally I have done something good, Satan is nevertheless able to turn it around in such a way that my holiness is reduced to nothingness. Then I make haste to seize hold of the article of the forgiveness of sins through Jesus Christ, who died and rose again for my sins [I Cor. 15:31]; and this is precisely what Satan does not want to let into my heart. But what does go into the heart is that I have done this and not done that, that I have given alms, been good, etc., just as I can say of our beloved prince that he had a faithful heart, devoid of malice and envy.
In other words, if Satan can’t get us to despair about our many sins, he tries to get us to be proud of our obedience. Again, Luther:
But by all means take care not to let anybody persuade you of this on your deathbed; for then the devil is not far away; he can throw in your face a little sin which reduces all such fine virtues to nothing, so that finally you come to such a pass that you say: Devil, rage as much as you please, I do not boast of my good works and virtues before our Lord God at all, nor shall I despair on account of my sins, but I comfort myself with the fact that Jesus Christ died and rose again, as the text here says.
Lo, when I believe this with my whole heart, then I have the greatest treasure, namely, the death of Christ and the power which it has wrought, and I am more concerned with that than with what I have done. Therefore, devil, begone with both my righteousness and my sin. If I have committed some sin, go eat the dung; it’s yours. I’m not worrying about it, for Jesus Christ died. St. Paul bids me comfort myself with this, that I may learn to defend myself from the devil and say: Even though I have sinned, it doesn’t matter; I will not argue with you about what evil or good I have done. There is no time to talk of that now; go away and do it some other time when I have been a bad boy, or go to the impenitent and scare them all you please. But with me, who have already been through the anguish and throes of death, you’ll find no place now. This is not the time for arguing, but for comforting myself with the words that Jesus Christ died and rose for me. Thus I am sure that God will bring me, along with other Christians, with Christ to his right hand and carry me through death and hell.
Martin Luther, Luther’s Works, vol. 51, p. 241.