20. He smote the rock, and the waters gushed out, and streams overflowed. That is, “We confess that He suffered and was struck, and that thence many people have flowed to Him.” The Jews neither could nor can deny that Christ suffered and that so many people have flowed to Him, and that streams of the most devout martyrs have been born from faith in Him. But in spite of that they still do not believe. For they are stubborn in their demand and are still looking for food for their desires, that is, they want to hear a teaching about God in line with their carnal desires, and they say: “Though we see that these things are so, does it follow that He is God? Not at all. For He cannot give us our food. If He could, He would. Therefore, ‘Can He give bread or prepare meat for His people?’ ” It is as if they were saying: “It seems that He cannot. For He does not give to us who worship God. If He were God, He would give to us, who certainly worship God. But He does not give to His worshipers, for they are all in poverty and lack the things that are after the flesh, constantly tossed about by the cross and the evils of the world.” Behold, behold, this is because to the Jews demanding a sign Christ is preached as a stumbling block. For since Christ gives carnal food neither to them nor to us, that is, lets them think, teach, and hear about Christ and His kingdom and glory in a carnal way, they do not believe in Him but tempt Him to the present day. And it is believable that anciently it was done similarly in a figure that those who tasted the manna easily put up with the poverty of the flesh and admonished those who were impatient likewise to hope in the Lord. But they burst forth with: “What are you saying? If God were with us, He would certainly provide for us. But how shall we now believe, when He can provide table and meat neither for you nor for us? If He could, why would He not do it?” And so they fell into doubt and lack of trust, yes, from impatience into unbelief, just as they are doing today.
Luther’s works, vol. 11: First Lectures 2