It is the same with the other Commandments. If you have a wife, a daughter, or a maid, you would not want her to be corrupted or to acquire a bad reputation. You want everyone to respect her, to treat her well, and to speak the best about her. Then why are you so perverse that you yearn for someone else’s wife and want to corrupt her yourself? Why do you not help to improve her reputation, instead of finding pleasure in talking behind her back and slandering her? Similarly, you would not want anyone to do you injury or harm, to malign you, or to do anything like that. Then why do you yourself violate the rule and standard that you demand of others and want them to keep? How can you judge, criticize, and condemn someone else if he does not treat you that way? Why do you refuse to obey your own rule? Go through all the commandments of the Second Table this way, and you will find that this is really the summary of all possible sermons, as He Himself says here. Thus this is properly termed a short sermon. But on the other hand, if it were expanded through all the details it implies, it is such a long sermon that it would be endless, since the things that will be done on earth until the Last Day are innumerable. It takes a good teacher to condense and summarize such a long-drawn-out sermon in such a way that everyone can carry it home with him, be reminded of it daily, and see what is missing in his whole life; for he has it written in his own heart, in fact, in his whole life and activity, as we shall hear in more detail.
I am convinced, moreover, that it would be influential and productive of fruit if we only got into the habit of remembering it and were not so lazy and inattentive. I do not regard anyone as so coarse or so evil that he would shirk this or be offended at it if he really kept it in mind. It was certainly clever of Christ to state it this way. The only example He sets up is ourselves, and He makes this as intimate as possible by applying it to our heart, our body and life, and all our members. No one has to travel far to get it, or devote much trouble or expense to it. The book is laid into your own bosom, and it is so clear that you do not need glasses to understand Moses and the Law. Thus you are your own Bible, your own teacher, your own theologian, and your own preacher. The way He directs you, you only need one look at them to find out how the book pervades all your works and words and thoughts, your heart and body and soul. Just guide yourself by this, and you will be more wise and learned than all the skill and all the books of the lawyers. (Luther’s Works, v. 21, pages 236-237).