15 “You have dried up the rivers of Etham”....What is Etham? For the word is Hebrew. What is Etham interpreted? Strong, stout. Who is this strong and stout one, whose rivers God dries up? Who but that very dragon? For “no one enters into the house of a strong man that he may spoil his vessels, unless first he shall have bound fast the strong man.” This is that strong man on his own virtue relying, and forsaking God: this is that strong man, who says, “I will set my seat by the north, and I will be like the Most High.” Out of that very cup of perverse strength he has given man to drink.
Strong they willed to be, who thought that they would be Gods by means of the forbidden food. Adam became strong, over whom was reproachfully said, “Behold, Adam has become like one of us.”...As though they were strong, “to the righteousness of God they have not been made subject.” Observe ye that a man has put out of the way his own strength, and remained weak, needy, standing afar off, not daring even to raise his eyes to Heaven; but smiting his breast, and saying, “O Lord, be merciful to me a sinner.” Now he is weak, now he confesses his weakness, he is not strong: dry land he is, be he watered with fountains and torrents.
They are as yet strong who rely on their own virtue. Be their rivers dried up, let there be no advancement in the doctrines of the Gentiles, of wizards, of astrologers, of magic arts: for dried up are the rivers of the strong man: “You have dried up the rivers of Etham.” Let there dry up that doctrine; let minds be flooded with the Gospel of truth.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)