Matthew 7:12. So whatever you wish that men would do to you, do so to them; for this is the Law and the Prophets.
With these words He concludes the teaching He has been giving in these three chapters, and wraps it all up in a little package where it can all be found. Thus everyone can put it in his bosom and keep it; it is as if He were saying: “Would you like to know what I have been preaching, and what Moses and all the Prophets teach you? I shall tell it to you so briefly and put it in such a way that you dare not complain about its being too long or too hard to remember.” This is the kind of sermon that can be expanded or contracted; from it all teaching and preaching go forth and are broadcast, and here they come back together. How could it be put more succinctly and clearly than in these words? The trouble is that the world and our old Adam refuse to let us ponder what He says and measure our lives against the standard of this teaching. We let it go in one ear and out the other. If we always measured our lives and actions against this standard, we would not be so coarse and heedless in what we do, but we would always have enough to do. We could become our own teachers, teaching ourselves what we ought to do; and we would not have to chase after holy lives and holy works, nor would we need many lawyers and law books. This is stated briefly and learned easily, if we only were diligent and serious in acting and living according to it.
Let me illustrate it with a somewhat crude example. Surely there is no one who would enjoy being robbed; if he asks his own heart about this, he has to say that he really would not enjoy it at all. Now, why does he fail to draw the conclusion that he should treat others the way he wants to be treated? At a market, you see everyone marking up his prices as high as he pleases, asking 30 pfennigs for something that is not worth If you ask him, “Friend, would you want to be treated that way?” he must be honest and reasonable enough to say: “I would be willing to pay for it what its market value is and what would be reasonable and proper, so that I am not overcharged.” There, you see, your heart is telling you honestly how you would like to be treated. And your conscience is arguing that you should treat others the same way; it can teach you well about your relations with your neighbor in buying and selling and all kinds of business, all things belonging to the Seventh Commandment (Ex. 20:15): “You shall not steal.” (Luther’s Works, v. 21, pages 235-236).