Luther knew from experience that the proclamation of Law and Gospel doesn't always produce love, peace, and harmony in congregations or communities. Here, for example, are some of the points he made about the Gospel causing conflicts:
For if we cling to our own thoughts and feelings, we are dismayed and hurt to learn that for our service, help, counsel, and kindness to the world and to everyone we should get no thanks except the deepest and bitterest hatred and cursed, poisonous tongues. If flesh and blood were in charge here, it would soon say: “If I am to get nothing else out of this, then let anyone who wants to, stick with the Gospel and be a Christian! The world can go to the devil for help if that is what it wants!”
This is the reason for the general complaint and cry that the Gospel is causing so much conflict, strife, and disturbance in the world and that everything is worse since it came than it was before, when things moved along smoothly, when there was no persecution, and when the people lived together like good friends and neighbors.
But here is what it says: “If you do not want to have the Gospel or be a Christian, then go out and take the world’s side. Then you will be its friend, and no one will persecute you. But if you want to have the Gospel and Christ, then you must count on having trouble, conflict, and persecution wherever you go.” Reason: because the devil cannot bear it otherwise, nor will he stop egging people on against the Gospel, so that all the world is incensed against it. Thus at the present time peasants, city people, nobles, princes, and lords oppose the Gospel from sheer cussedness, and they themselves do not know why.
So this is what I say in reply to these idle talkers and grumblers: “Things neither can nor should run peacefully and smoothly. How could things run smoothly, when the devil is in charge and is a mortal enemy of the Gospel? There is good reason for this, too, since it hurts him in his kingdom, where he can feel it. If he were to let it go ahead unhindered, it would soon be all over and his kingdom would be utterly destroyed. But if he is to resist it and hinder it, he must rally all his art and power and arouse everything in his might against it.
So do not hope for any peace and quiet so long as Christ and His Gospel are in the midst of the devil’s kingdom. And woe upon the peaceful and smooth situation that used to be, and upon those who would like to have it back! This is a sure sign that the devil is ruling with all his might and that no Christ is there. I am worried that it may be this way again and that the Gospel may be taken away from us Germans all too soon, which is just what these rioters are struggling for now.”
But we have the assurance that it is not our fault when there is trouble. It would give us heartfelt joy if everything went right. We have done our share. We have been teaching, warning, pleading, beseeching, and giving in, even to our enemies, offering them peace, and doing everything we should do. We have given help and counsel with all our might, at our own risk and disadvantage, tolerating what we should. Yet all we accomplish is shameful and poisonous persecution, slander, and abuse from men who will not stop till they have cooled their rage in our blood. Since the situation will never be different, we let them go ahead with their threatenings, fury, and blasphemy, and hold on to the comfort we have heard. We are sure that they cannot accomplish what they desire until they first topple Christ from heaven and make a liar out of Him, with all that He has said.
For so men persecuted the prophets who were before you.
Luther on Matthew 5:13, vol. 21 of Luther's Works