The holiness of the word and the purity of doctrine are powerful and sure, so that even if Judas, Caiaphas, Pilate, the pope, Harry, or the devil himself preached it, or baptized truly (purely, without addition), they would still receive the same, pure word and the true, holy baptism, for there must always be hypocrites and false Christians in the church and a Judas among the apostles. Again, the impurity of doctrine that is not or is without God’s word is such a poisonous evil that even if St. Peter, indeed, an angel from heaven, were to preach it, he would nevertheless be accursed, Galatians I [:8]. Therefore those who teach, baptize, or distribute the sacrament falsely cannot be or remain in the church, as Psalm 1 [:5] says. For they act not only against the life the church must endure—particularly when it is hidden—but also against the doctrine that must gleam and shine in public to be a guide for life. This has been taught from the beginning, as St. John says, “They went out from us, but they were not of us” [I John 2:19], and, “They are in the church but not of the church”; or, “In number but not in merit,” and the like. Accordingly, we draw this distinction: not all are Christians who pretend to be Christians. But when there is disagreement in doctrine, it becomes quite evident who the true Christians are, namely, those who have God’s word in purity and refinement. (Luther’s Works, v.41, p.218-219)

Now the purpose of all this is to show that the church must teach God’s word alone, and must be sure of it. The church is the pillar and bulwark of the truth, built on the rock, and called holy and irreproachable [Eph. 2:21]. Thus one rightly and truly says, “The church cannot err, for God’s word which it teaches cannot err.” But whatever else is taught or whatever is not with certainty God’s word, cannot be the doctrine of the church, but must be the doctrine, falsehood, and idolatry of the devil. The devil cannot say (since he is a liar and the father of lies), “God says this.” But, as Christ says in John 8 [:44], he must speak ex propriis, “from and of himself,” that is, he must lie; and so, without God’s word, all his children speak from and of themselves, that is, they must lie. (Luther’s Works, v.41, p.217)

Now observe, my dear friend, what a wonderful thing this is: we who teach God’s word with such certainty are so weak, and in our great humility so timid, that we do not like to brag that we are God’s church, witnesses, servants, and preachers, and that God speaks through us, etc. And yet, we do not doubt it, since we are certain to have and to teach his word. Such timidity stems from the fact that we earnestly believe God’s word is such a noble and majestic thing that we regard ourselves far too unworthy of its being spoken and done through us who are still creatures of flesh and blood. But our adversaries, the devils, the papists, the sects, and all the world, are confident and intrepid, asserting with great holiness and arrogance, “Here is God, we are God’s church, his servants, prophets, and apostles.” It is just like every false prophet has always done, so that even Harry-wurst dares to boast that he himself is a Christian prince. But the true sign of the true holy church has always been humility and fear in the word of God, whereas lust and wantonness in innovating have been the true signs of the devil—something that becomes quite evident from the pope’s decretals. (Luther’s Works, v.41, p.217-218)

This we say about doctrine, which must be pure and clean, namely, the dear, blessed, holy, and one word of God, without any addition. But life, which should daily direct, purify, and sanctify itself according to doctrine, is not yet entirely pure or holy, so long as this maggoty body of flesh and blood is alive. But as long as it is in the process of purification and sanctification, being continually healed by the Samaritan and no longer decaying in its own impurity, it is graciously excused, pardoned, and forgiven for the sake of the word, through which it is healed and purified; thus it must be called pure. This is why the holy Christian church is not a whore or unholy, because it continues to hold to and remain with the word (which is its holiness) without blemish and with strength. “You are already made clean (says Christ in John 15 [:3]) by the word which I have spoken to you,” not on your own account. (Luther’s Works, v.41, p.218)