[22] Also St. Paul in Rom. 12:7 says: "Whether prophecy, let us prophesy according to the proportion of our faith." That is, all doctrines must harmonize and agree with faith, so that nothing be taught but faith. He who has not this faith cannot judge of any doctrine correctly, for here the carnal minded who are smart and worldly wise are incompetent, only pious spiritual hearts can do that...
[25] Therefore learn well to be on your guard, and thoroughly examine all doctrines, whether they rightly teach, and apply this chief and fundamental doctrine, that is, whether they preach Christ correctly and purely. For you will certainly find among all factions and sects, that they always either omit this article entirely, or bring something with it to which they lead the people as though that were particularly necessary to salvation.
From Luther's Church Postil, "Eighth Sunday After Trinity. Third Sermon: Matt. 7:15-23. Concerning False Teachers"
