Faith Depends on God's Word

The Word and faith should stand together, for the one cannot exist without the other. He who believes, but does not have the Word, believes as do the Turks and the Jews. They believe that God is gracious and merciful, but they lack a promise; for God will not be gracious without Christ. Just so the Word does no good to the man who has it but lacks faith.

Thus the Word and faith have been given in marriage, and neither can let itself be separated from the other. An enthusiast believes that Judgment Day will come within four weeks. But such a faith is a plain lie, for it rests on no word of God. A Turk believes that he will come to enjoy salvation through the guidance of his Mohammed. But that is a plain lie, for this notion rests on no word of God. Again the pope believes that a Christian must advance to heaven by his own works. But this is a false faith, for it lacks the word and promises of God. Therefore a man may well have faith, that is, he may expect and hope for something; but since it lacks the basis of the Word, it is no real faith.... 

But we Christians do not lack the word, for by God's grace we actually have the Word in all its purity. But we lack faith. Therefore we are not able to believe the Word as firmly as we should. And yet the others can believe firmly and strongly without the Word. The devil is the cause of this, and original sin; for it draws us from the Word and the truth to lies. Therefore it is difficult to guard against unbelief, even though we have the Word; for our flesh and reason will have none of the Word.

Source: Quoted in Ewald M. Plass, What Luther Says, A Practical In-home Theology for the Active Christian. St. Louis: Concordia Publishing House. 1959 Edition, 10th Printing (1994), p. 471