… Christ says: “He that heareth not the words of God is not of God, but is the devil’s child.” The most deplorable circumstance in this connection is the infatuation, in which these defamers of God’s Word live, so that they do not perceive the error of their ways, but defend their conduct with bitter resolution. Thus did the Jews; they answered Christ: “Say we not well that Thou art a Samaritan, and hast a devil? A self-righteous devil, who thinks that he is always right, is the worst of all devils; and such an one was prompting the Jews. This is the first lesson taught in our Gospel, that they who will not hear the Word of God are not of Him, but of the devil. This, of course, does not mean that the devil created such persons with their body, mind and all they possess. No, God bestowed upon them these gifts, wherefore we must distinguish between the abilities of man and their use. The man who lies and cheats has a good tongue given to him by God, but the use which he makes of it is of the devil in whose service he abuses it against God. Thus also God may give to man beautiful and charming eyes, but their employment for lustfulness is of the devil. If the heart yearns after lewdness, falsehood and deceit, it is wicked and of the devil, in respect to its use, though it be good and of God as to its essence.
On the other hand, “to be of God” means to employ willingly and gladly the ears in hearing the Word of God, and to accept rebuke for that which is wrong. Thus we should also use the tongue to pray, to preach, to instruct and to give consolation. Ears and tongues thus engaged are of God and good; they accomplish the purpose for which they were given. The heart also, if it meditates upon that which is comely, upon what is beneficial for our fellow men, and does not give offense, is a creature of God as well as the ears and the tongue. It is of God, because it strives to follow the precepts of God, and loves to hear His Word, and shuns everything opposed to it. It may happen, alas! that we are overtaken unaware by our weakness; that we curse where we ought to pray, and give way to anger where we ought to manifest kindliness; but if we see our wrong and confess it, asking for mercy, we may then indeed have wavered, or even fallen, but we are not yet of the devil. If a man repents of his sins, and turns to God with the firm resolution to avoid that which is wrong, he will obtain mercy, and the devil is not his master. (Luther’s House Postil, Sermon for Judica, Volume 1, p. 296-297)