If the word were to be sinful or untrue, after what would or could men guide their lives? Then, no doubt, a blind man would lead a blind man, and both would fall into the pit [Matt. 15:14]. If the plumb line or the T-square were false or crooked, what kind of work would or could the master-builder produce? One crooked thing would make the other crooked, without limit or measure. Life too can be sinful and untrue in the same way—unfortunately life is indeed very untrue—but doctrine must be straight as a plumb line, sure, and without sin. Therefore nothing must be preached in church except the sure, pure, and one word of God. Where that is missing, we no longer have the church, but the synagogue of the devil [Rev. 2:9], just as a godly wife (an example the prophets always use) should not listen to any other word in her house or in her bed than that of her husband, and if she does listen to the word of someone who does not belong in her husband’s bed, she is certainly a whore. (Luther’s Works, v.41, p.216-217)

It is, of course, quite true that if judged by its way of life, the holy church is not without sin, as it confesses in the Lord’s Prayer, “Forgive us our trespasses”; and John writes [I John 1:8, 10], “If we say we have no sin, we lie and make God a liar, who calls us all sinners”—also Romans 3 [:23], Psalm 14 [:3] and 51 [:7]. But doctrine should not be sinful or reproachable. It does not belong to the Lord’s Prayer and its petition, “Forgive us our trespasses,” because it is not something we do, but is God’s own word, which cannot sin or do wrong. A preacher should neither pray the Lord’s Prayer nor ask for forgiveness of sins when he has preached (if he is a true preacher), but should say and boast with Jeremiah, “Lord thou knowest that which came out of my lips is true and pleasing to thee” [Jer. 17:16]; indeed, with St. Paul and all the apostles and prophets, he should say firmly, Haec dixit dominus, “God himself has said this” [I Cor. 1:10]. And again, “In this sermon I have been an apostle and a prophet of Jesus Christ” [I Thess. 4:15]. Here it is unnecessary, even bad, to pray for forgiveness of sins, as if one had not taught truly, for it is God’s word and not my word, and God ought not and cannot forgive it, but only confine, praise, and crown it, saying, “You have taught truly, for I have spoken through you and the word is mine.” Whoever cannot boast like that about his preaching, let him give up preaching, for he truly lies and slanders God. (Luther’s Works, v.41, p.216)

Now a kindhearted man (as they say) might ask, “What harm is there if one holds to the word of God and yet lets all these matters, or at least those that are bearable, remain as well?” I answer, “They may be called kindhearted, but they are wrong-hearted and misled, for you have heard that it is impossible to teach any word other than God’s word, to serve anyone other than God, to light any light other than that which has been placed by God in the darkness” [Matt. 6:24]. It is indeed an error and a will-o’-the-wisp thing, even if it were only one single thing, for the church ought not and cannot teach lies and error. If it teaches one lie, then it is wholly false, as Christ says in Luke 11 [:35–36], “Be careful lest the light in you be darkness. If then your whole body is full of light, having no dark part, it will be wholly bright.” That is to say, it must be all light, without any darkness in it. The church must teach God’s word and truth alone, and not error or falsehood. And how could it be otherwise? For God’s mouth is the mouth of the church, and vice versa. God cannot lie, nor can the church. (Luther’s Works, v.41, p.215-216)