A Christian, you see, has to learn that the devil can quote the truth and extol God’s Word just as well as pious Christians. Whoever does not learn this is easily deceived and let astray, and then it is all over with him. For the person who allows himself to be fooled by sheep’s clothing, him the wolf tears to pieces. That is why we need to say, I want gladly to hear God’s Word and the truth; but before I rely too much on what I hear, I want to see whether it rhymes with my catechism and the preaching which I have heard up till now. The person, therefore, who is on his toes and not naively gullible, but searches the Word, that person has no problem. But the person who is lackadaisical about his catechism and the doctrine he has previously learned, with ears only for every whimsical fanatic, that person is done for.
Martin Luther, Second House Postil for Trinity 8 (Matthew 7:15-23)