"When [sin] reigns in one’s nature, not yet fully purged, then assuredly the true doctrine is soon lost"

Martin Luther was not one to mince words. 

In one his last sermons, on Rom. 12:3, Luther stated about a month before his death (AE 51:376-377):

“Therefore, see to it that you hold reason in check and do not follow her beautiful cogitations. Throw dirt in her face and make her ugly. Don’t you remember the mystery of the holy Trinity and the blood of Jesus Christ with which you have been washed of your sins? Again, concerning the sacrament, the fanatical antisacramentalists say, ‘What’s the use of bread and wine? How can God the Almighty give his body in bread?’ I wish they had to eat their own dirt. They are so smart that nobody can fool them. If you had one in a mortar and crushed him with seven pestles his foolishness still would not depart from him. Reason is and should be drowned in baptism, and this foolish wisdom will not harm you, if you hear the beloved Son of God saying, ‘Take, eat; this is my body, which is given for you; this bread which is administered to you, I say, is my body.’ If I hear and accept this, then I trample reason and its wisdom under foot and say, ‘You cursed whore, shut up! Are you trying to seduce me into committing fornication with the devil?’ That’s the way reason is purged and made free through the Word of the Son of God.

So let us deal with the fanatics as the prophets dealt with the spiritual harlots, the idolaters, the wiseacres, who want to do things better than God does. We should say to them, ‘I have a Bridegroom, I will listen to him. Your wisdom is utter foolishness. I destroy your wisdom and trample it under foot.’ This struggle will go on till the last day. This is what Paul [in Rom. 12:3] wants; we are to quench not only the low desires but also the high desires, reason and its high wisdom. When whoredom invades you, strike it dead, but do this far more when spiritual whoredom tempts you. Nothing pleases a man so much as self-love, when he has a passion for his own wisdom. The cupidity of a greedy man is as nothing compared with a man’s hearty pleasure in his own ideas. He then brings these fine ideas into the Scriptures, and this is devilishness pure and simple. This sin is forgiven, but when it reigns in one’s nature, not yet fully purged, then assuredly the true doctrine is soon lost, however willingly one preaches and willingly one listens. Then Christ is gone. Then they fall down before the devil on the mountain and worship him (Matt. 4 [:8–10]).”

Also note this quote:

AE 24:246: “It does not require such great skill to begin to love; but, as Christ says here, remaining in love takes real skill and virtue. In matrimony many people are initially filled with such ardent affection and passion that they would fairly eat each other; later they become bitter foes. The same thing happens among Christian brethren. A trivial cause may dispel love and separate those who should really be bound with the firmest ties; it turns them into the worst and bitterest enemies. That is what happened in Christendom after the days of the apostles, when the devil raised up his schismatic spirits and heretics, so that bishops and pastors became inflamed with hatred against one another and then also divided the people into many kinds of sects and schisms from which Christendom suffered terrible harm. That is the devil’s joy and delight. He strives for nothing else than to destroy love among Christians and to create utter hatred and envy. For he knows very well that Christendom is built and preserved by love. In Col. 3:14 Paul speaks of love as ‘binding everything together in perfect harmony.’ And in 1 Cor. 13:13 he calls love the greatest virtue, which accomplishes and achieves most in the Christian realm. For in the absence of love doctrine cannot remain pure; nor can hearts be held together in unity.”

-Quotes obtained from the materials given out at the 27th Annual Lutheran Free Conference: “The Character of Christian Worship: It May Not Be What You Think” – Saturday, October 25th, 2014 at Redeemer Lutheran Church in St. Cloud, MN.  Full audio available here.