13 What more after the heads of dragons? For those dragons have their chief, and he is himself the first great dragon. And concerning him what has He done that has wrought Salvation in the midst of the earth? Hear: “You have broken the head of the dragon”. Of what dragon? We understand by dragons all the demons that war under the devil: what single dragon then, whose head was broken, but the devil himself ought we to understand? What with him has He done? “You have broken the head of the dragon.” That is, the beginning of sin. That head is the part which received the curse, to wit that the seed of Eve should mark the head of the serpent. For the Church was admonished to shun the beginning of sin. Which is that beginning of sin, like the head of a serpent? The beginning of all sin is pride. There has been broken therefore the head of the dragon, has been broken pride diabolical. And what with him has He done, that has wrought Salvation in the midst of the earth? “You have given him for a morsel to the Ethiopian peoples.” What is this? How do I understand the Ethiopian peoples? How but by these all nations? And properly by black men: for Ethiopians are black. They are themselves called to the faith who were black; the very same indeed, so that there is said to them, “for you were sometime darkness, but now light in the Lord.”...Thence was also that calf which the people worshipped, unbelieving, apostate, seeking the gods of the Egyptians, forsaking Him who had delivered them from the slavery of the Egyptians: whence there was enacted that great Sacrament. For when Moses was thus angry with them worshipping and adoring the idol, and, inflamed with zeal for God, was punishing temporally, in order that he might terrify them to shun death everlasting; yet the head itself of the calf he cast into the fire, and ground to powder, destroyed, scattered on the water, and gave to the people to drink: so there was enacted a great Sacrament. O anger prophetic, and mind not perturbed but enlightened! He did what? Cast it into the fire, in order that first the form itself may be obliterated; piece by piece grind it down, in order that little by little it may be consumed: cast it into the water, give to the people to drink! What is this but that the worshippers of the devil had become the body of the same? In the same manner as men confessing Christ become the Body of Christ; so that to them is said, “but you are the Body of Christ and the members.” The body of the devil was to be consumed, and that too by Israelites was to be consumed. For out of that people were the Apostles, out of that people the first Church....Thus the devil is being consumed with the loss of his members. This was figured also in the serpent of Moses. For the magicians did likewise, and casting down their rods they exhibited serpents: but the serpent of Moses swallowed up the rods of all those magicians. Let there be perceived therefore even now the body of the devil: this is what is coming to pass, he is being devoured by the Gentiles who have believed, he has become meat for the Ethiopian peoples. This again, may be perceived in, “You have given him for meat to the Ethiopian peoples,” how that now all men bite him. What is, bite him? By reproving, blaming, accusing. Just as has been said, by way of prohibition indeed, but yet the idea expressed: “but if you bite and eat up one another, take heed that you be not consumed of one another.” What is, bite and eat up one another? You go to law with one another, you detract from one another, you heap revilings upon one another. Observe therefore now how that with these bitings the devil is being consumed. What man, when angry with his servant, even a heathen, would not say to him, Satan? Behold the devil given for meat. This says Christian, this says Jew, this says heathen: him he worships, and with him he curses!...
14. “You have cleft the fountains and torrents”: in order that they might flow with the stream of wisdom, might flow with the riches of the faith, might water the saltness of the Gentiles, in order that they might convert all unbelievers into the sweetness of the faith by their watering....In some men the Word of God becomes a well of water springing up unto life eternal; but others hearing the Word, and not so keeping it as that they live well, yet not keeping silence with tongue, they become torrents. For they are properly called torrents which are not perennial: for sometimes also in a secondary sense torrent is used for river: as has been said, “with the torrent of Your pleasures You shall give them to drink.” For that torrent shall not ever be dried up. But torrents properly are those rivers named, which in summer fail, but with winter rains are flooded and run. You see therefore a man sound in faith, that will persevere even unto the end, that will not forsake God in any trial; for the sake of the truth, not for the sake of falsehood and error, enduring all difficulties. Whence is this man so vigorous, but because the Word has become in him a well of water springing up unto life eternal? But the other receives the Word, he preaches, he is not silent, he runs: but summer proves whether he be fountain or torrent. Nevertheless through both be the earth watered, by Him who has wrought Salvation in the midst of the earth: let the fountains overflow, let the torrents run.
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)