Therefore when He shows that man does not live by bread alone, this pertains not only to the manna but to both conditions, namely, both to the affliction of hunger and to the abundance of manna. Thus the meaning is: God wanted to show you this goodness of His through your own experience, that in hunger you might learn to believe the Word of Him who promised, be sustained by it in hunger and not perish, and then finally by this faith receive the satisfaction of your bodily needs. All this He does in order to remove from you the idol of your belly and to teach you that the life of man does not rest in this, that the belly is provided with its bread, but that both parts belong to it, namely, soul and body. This you would never learn if your belly were always satisfied and you never learned through the Word to trust in God while your belly is in need.
In this way Christ quotes this passage in Matt. 4:4, when Satan holds before Him the care of the belly alone. Christ says: “Not by bread alone does man live,” not denying that man lives by bread, but saying that it is not solely by bread. For if bread is lacking, then he lives through the Word. When he has this by faith, then the bread must follow, even if it has to be produced from rock, or, as here, sent from heaven. Therefore when he says: “He afflicted you with famine, and tempted,” he calls them away from the belly and the care of the belly, to teach them in the midst of hunger to trust in and live by the Word. When he says: “And He gave you manna,” he shows that bread will not be lacking for those who by the Word survive in hunger. You see, therefore, that nothing but faith is taught by these words. By it we hold onto God and believe that we “have God,” as the First Commandment says.3
Luther’s works, vol. 9: Lectures on Deuteronomy 1525