Those who want to be pious, who fear God, and who think about how to live and behave, must know that they simply have no right to do business with their property and manage it as they please, as though they themselves were the lords of all. They have the obligation to carry on their business in a proper and orderly way; this is why there is territorial and civil law. That is how everyone would want his neighbor to treat him; therefore he should also treat his neighbor that way, taking and offering only good merchandise. Christ means this commandment seriously, and He will not let it be made free or optional, as though one could obey it or disobey it with impunity. He will enforce it, too, however much the world may take it as an insult and despise it. If you do not obey it, He will deal with you according to your own standard and judgment, and it will strike you in your house and home. You will have no blessing from what you have acquired in disobedience to this teaching, but you and your children will have only trouble and sorrow. He wants His commandment to be kept; otherwise you will have neither property nor good fortune.
In the second place, Christ not only makes this so intimate, as we have said, that we have to see it in everything we look at, He also portrays it in such a way that everyone has to blush in shame over himself. There is no one who enjoys doing something wrong when other people can see it. No one dares to sin publicly, in the presence of people, with the same freedom as he does privately, where no one can see him. So Christ intends here to appoint us as our own witnesses and to make us afraid of ourselves. Then if we do something wrong, our conscience will stand up against us with this commandment, as an eternal witness, and say: “Look here, what are you doing? According to the usual fair-business practice, you ought to put such and such a price on this. But you are putting on a much higher price. Or the way you are debasing and misrepresenting this merchandise, you would not want to have someone else sell you something like that.” How it would annoy you if someone charged you a gulden for something barely worth ten groschen! If you had one drop of honest blood in your body, you would have to be ashamed of yourself. If someone else acted this way, you would call him a thief and a villain. Then why are you not ashamed of yourself, since it is not someone else but yourself who has to make this accusation, and you are condemned by your own conscience? This may be all right for a brazen hardhead, who has no sense of shame before the people or before himself, much less before God. But when someone else treats you that way you can quickly exclaim: “Is it not a sin and a shame and a clever18 way of robbing my wallet?” You can quickly recognize a thief and a villain in someone else; but you refuse to see the one working in your own breast, whom you can easily catch and feel. (Luther’s Works, v. 21, pages 237-239).