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)