39 Do you wish to see how incapable he is of hurting you, unless permitted? “These,” he says, “wait all upon You, that You may give them meat in due season”. And this serpent wishes to devour, but he devours not whom he wishes....You have heard what the serpent's meat is. Thou dost not wish that God give you to be devoured by the serpent; because not the serpent's food: i.e. forsake not the Word of God. For where it is said to the serpent, “Dust you shall eat,” it is said to the transgressor, “Dust you are, and unto dust you shall return.” Thou dost not wish to be the serpent's food?
Be not dust. How, you reply, shall I not be dust? If you have not a taste for earthly things. Hear the Apostle, that you may not be dust. For the body which you wear is earth: but do thou refuse to be earth. What means this? “Set your affection on things above, not on things on the earth.” If you dost not set your affections on earthly things, you are not earth: if you are not earth, you are not devoured by the serpent, whose appointed food is earth. The Lord gives the serpent his food when He will, what He will: but He judges rightly, he cannot be deceived, He gives him not gold for earth. “When You have given it them, they gather it.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)