37 He then, my brethren, who does wish to watch the serpent's head, and safely to pass this sea; for it must be that this serpent dwells here, and, as I had commenced saying, the devil when he fell from heaven received this region; let him watch his head, on the part of the fear of the world, and of the lusts of the world. For it is hence that he suggests some object of fear or of desire; he tries your love, or your fear. If you fear hell, and lovest the kingdom of God, you will watch his head....“There is no power but of God.” What then do you fear? Let the dragon be in the waters, let the dragon be in the sea: you are to pass through it. He is made so as to be made sport of, he is ordained to inhabit this place, this region is given him. You think that this habitation is a great thing for him, because you know not the dwellings of the angels whence he fell: what seems to you his glory, is his damnation.
38....What then do you fear? Perhaps he is about to try your flesh: it is the scourge of your Lord, not the power of your tempter. His wish is to injure that salvation which is promised: but he is not allowed: but that he may not be allowed, have Christ for your Head: repel the serpent's head: consent not unto his suggestion, slip not from your path. “There is that Leviathan, whom You have made to make sport of him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)