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)