Matthew 5:25. Make friends quickly with your accuser, while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison;
26. truly, I say to you, you will never get out till you have paid the last penny.
In the preceding verses He was preaching against the man who injured his neighbor or became angry with him. Now He talks about how the injured party should behave. He stays with the analogy He had used earlier, that is, the procedure in a courtroom with two parties opposed to each other, one party as the accuser and the other party as the accused, and with the judge pronouncing sentence and punishing the guilty party. What He wants to say is that while a man who injures another should in a friendly way become reconciled with him, the injured party should also be willing to be reconciled and to forgive. This, too, is a delicate problem, where it is easy for many people to cover up and decorate their malice by saying that they are willing to forgive but not to forget. They can always use the excuse I have mentioned, that it is all right to be angry at wickedness. Hence they suppose that they have good reason and justification for doing so.
That is why He issues another warning here and shows that this commandment not only forbids wrath, but also requires a person cheerfully to forgive and forget injury that he has suffered. That is what God has done with us and continues to do when He forgives sin: He expunges it from the record and no longer remembers it. Still it is neither necessary nor possible for a man to forget it in the sense that he never thinks of it again, but in the sense that your heart is just as friendly toward your neighbor as it used to be before he injured you. If the stump remains in your heart and you are not as friendly and kind toward him as you used to be, that is not forgetting or even cordially forgiving. You are still the scoundrel who comes before the altar with his sacrifice and tries to serve God even while his heart is crammed full of anger, envy, and hate. But very few people pay any attention to this at all. They all walk around in their beautiful mask; they fail to see the relation of their heart to this commandment, which summarily rejects any anger or ill will against the neighbor.
Of course, as we have said, anger is sometimes necessary and proper. But be sure that you use it correctly. You are commanded to get angry, not on your own behalf, but on behalf of your office and of God; you must not confuse the two, your person and your office. As far as your person is concerned, you must not get angry with anyone regardless of the injury he may have done to you. But where your office requires it, there you must get angry, even though no injury has been done to you personally. For example, a pious judge gets angry with a criminal, even though personally he wishes him no harm and would rather let him off without punishment. His anger comes out of a heart where there is nothing but love toward his neighbor. Only the evil deed is punishable and must bear the anger; without it there would be no anger or punishment. But if your brother has done something against you and angered you, and then begs your pardon and stops doing wrong, your anger, too, should disappear. Where does the secret spite come from which you continue to keep in your heart? The deed that caused your anger is gone, and in its place have come other deeds, which show that the man is converted and has become a completely different person, a new tree with new fruits. Now he gives you his love and his highest esteem, he blames and reproaches himself on your account. If you do not give him another chance and cordially forgive him, you must really be a scoundrel before both God and the world; and you deserve the sentence which Christ threatens here.
Luther, M. (1999, c1956). Vol. 21: Luther's works, vol. 21 : The Sermon on the Mount and the Magnificat (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Luther's Works (21:82). Saint Louis: Concordia Publishing House.