But suppose you could truthfully say, “There was no false teaching in my sermon.” Nevertheless your entire sermon may have been wrong. Can that be true? The second part of the thesis states: “If you wish to be an orthodox teacher, you must rightly distinguish Law and Gospel.”
This is the litmus test of a proper sermon. The value of a sermon depends not only on whether it is in agreement with the same but also on whether Law and Gospel have been rightly distinguished. If the same building materials are provided to two different architects, sometimes one will construct a magnificent building, while the other, using the same materials, will make a mess of it. Because he is dim-witted, the latter may want to begin with the roof, or place all the windows in one room, or stack layers of stone or brick in such a way that the wall will be crooked. One house will be out of plumb and such a bungled piece of work that it will collapse, while the other will stand firm and be a habitable and pleasant place to live. In like manner, two different sermons might contain all the various doctrines-and while the one sermon may be a glorious and precious piece of work, the other may be wrong throughout.
Law & Gospel: How to Read and Apply the Bible by C. F. W. Walther (A Reader’s Edition) p 37