This also must be noted, that as the Word of God appears to the carnal to be lacking much and in many things, so it appears to the spiritual to abound in all things like many spoils. As in the figure of the children of Israel, who seemed to be lacking everything in the wilderness, so that even the manna was sickening to them, complaining that they were in a desert of barrenness, so it is necessary for the hearers of the Word of God to be led into the wilderness where there is nothing that the flesh demands. But when the flesh has been crucified and all that is of the flesh has been removed, only the spirit should be fed and led into the land of promise, that is, the spiritual man.
Therefore the carnal man necessarily murmurs here, because he finds nothing that pleases him. Thus the more carnal and worldly a man is, the more he has to bring up against the Word of God. And what wonder? Since it is altogether foolishness to him, he knows nothing useful in it. Therefore great is the praise of the church, which has the Spirit so abundantly that it esteems only the words of God, to rejoice in them above all spoils. Rejoicing in those things which to others seem to be sad things and nothings without a doubt puts a great void between the latter and the former. (Luther’s Works, v. 11, p.520-521)