15 But think not that those “fowls of heaven” follow their own authority; see what the Psalm says: “From the midst of the rocks they shall give their voice.” Now, if I shall say to you, Believe, for this said Cicero, this said Plato, this said Pythagoras: which of you will not laugh at me? For I shall be a bird that shall send forth my voice not from the rock. What ought each one of you to say to me? What ought he who is thus instructed to say? “If any one shall have preached unto you a gospel other than that you have received, let him be anathema.” What do you tell me of Plato, and of Cicero, and of Virgil? You have before you the rocks of the mountains, from the midst of the rocks give me your voice. Let them be heard, who hear from the rock: let them be heard, because also in those many rocks the One Rock is heard: for “the Rock was Christ.” Let them therefore be willingly heard, giving their voice from the midst of the rocks. Nothing is sweeter than such a voice of birds. They sound, and the rocks resound: they sound; spiritual men discuss: the rocks resound, testimonies of Scripture give answer. Lo! Thence the fowls give their voice from the midst of the rocks, for they dwell on the mountains.
16. “Watering the mountains from the higher places”. Now if a Gentile uncircumcised man comes to us, about to believe in Christ, we give him baptism, and do not call him back to those works of the Law. And if a Jew asks us why we do that, we sound from the rock, we say, This Peter did, this Paul did: from the midst of the rocks we give our voice. But that rock, Peter himself, that great mountain, when he prayed and saw that vision, was watered from above....
17. “From the fruit of Your works shall the earth be satisfied.” What is, “From the fruit of Your works”? Let no man glory in his own works: but “he that glories, let him glory in the Lord.” With Your grace he is satisfied, when he is satisfied: let him not say that grace was given for his own merits. If it is called grace, “it is gratuitously given;” if it is returned for works, wages are paid. Freely therefore receive, because ungodly you are justified.
18. “Bringing forth grass for the cattle, and green herb for the service of men”. This is true, I perceive; I recognise the creation: the earth does bring forth grass for the cattle, and green herb for the service of men. But I perceive the words, “You shall not muzzle the mouth of the ox which treads out the grain: Does God take care for oxen? For our sakes therefore the Scripture says it.” How then does the earth bring forth grass for the cattle? Because “the Lord has ordained that they which preach the Gospel should live of the Gospel.” He sent preachers, saying unto them, “Eat such things as are set before you of them: for the labourer is worthy of his hire.”...They give spiritual, they receive carnal things; they give gold, they receive grass....“If we have sown unto you spiritual things, is it a great matter if we shall reap your carnal things?” This the Apostle said, a preacher so laborious, so indefatigable, so well tried, that he gives this very grass to the earth. “Nevertheless,” he says, “we have not used this power.” He shows that it is due to him, yet he received it not; nor has he condemned those who have received what was due. For those were to be condemned who exact what is not due, not they who accept their recompense: yet he gave up even his own recompense. You do not cease to owe to another, because one has given up his dues, otherwise you will not be the watered earth which brings forth grass for the cattle.... You receive spiritual things, give carnal things in return: to the soldier they are due, to the soldier you return them; you are the paymaster of Christ. “Who goes a warfare any time at his own charges? Who plants a vineyard, and eats not of the fruit thereof? Or who feeds a flock, and eats not of the milk of the flock? I speak not thus, that it should be so done unto me.” There has been such a soldier as gave up his rations of food even to the paymaster: yet let the paymaster pay the rations....
19. “That it may bring forth bread out of the earth.” What bread? Christ. Out of what earth? From Peter, from Paul, from the other stewards of the truth. Hear that it is from the earth: “We have,” says St. Paul, “this treasure in earthen vessels, that the excellency of the power may be of God, and not of us.” He is the bread who descended from heaven, that He might be brought forth out of the earth, when He is preached through the flesh of His servants. The earth brings forth grass, that it may bring forth bread from the earth. What earth brings forth grass? Pious, holy nations. That bread may be brought forth out of what earth? The word of God out of the Apostles, out of the stewards of God's Sacraments, who still walk upon the earth, who still carry an earthly body.
20. “And wine makes glad the heart of man”. Let no man prepare himself for intoxication; nay, let every man prepare him for intoxication. “How excellent is Your cup which makes inebriate!” We choose not to say, Let no man be drunk. Be inebriated; yet beware, from what source. If the excellent cup of the Lord does saturate you, your ebriety shall be seen in your works, it shall be seen in the holy love of righteousness, it shall, lastly, be seen in the estrangement of your mind, but from things earthly to heavenly. “To make him a cheerful countenance with oil.”...What is the making the countenance cheerful with oil? The grace of God; a sort of shining for manifestation; as the Apostle says, “The Spirit is given to every man for manifestation.” A certain grace which men can clearly see in men, to conciliate holy love, is termed oil, for its divine splendour; and since it appeared most excellent in Christ, the whole world loves Him; who though while here He was scorned, is now worshipped by every nation: “For the kingdom is the Lord's, and He shall be Governor among the people.” For such is His grace, that many, who do not believe in Him, praise Him, and declare that they are unwilling to believe in Him, because no man can fulfil what He does command. They who with reproaches once raged against Him, are hindered by His very praises. Yet by all is He loved, by all is He preached; because He is excellently anointed, therefore He is Christ: for He is called Christ from the Chrism or anointing which He had. Messiah in the Hebrew, Christ in the Greek, Unctus in the Latin: but He anoints over His whole Body. All therefore who come, receive grace, that their countenances may be made glad with oil.
21. “And bread strengthens man's heart.” What is this, brethren? As it were, he has forced us to understand what bread he was speaking of. For while that visible bread strengthens the stomach, feeds the body, there is another bread which strengthens the heart, in that it is the bread of the heart....There is therefore a wine that truly makes glad the heart, and knows not to do anything else than to gladden the heart. But that you may not imagine that this indeed should be taken of the spiritual wine, but not of that spiritual bread; He has shown this very point, that it is also spiritual: “and bread,” he says, “strengthens man's heart.” So understand it therefore of the bread as thou dost understand it of the wine; hunger inwardly, thirst inwardly: “Blessed are they,” says our Lord, “who hunger and thirst after righteousness; for they shall be filled.” That bread is righteousness, that wine is righteousness: it is truth, Christ is truth. “I am,” He said, “the living bread, who came down from heaven;” and, “I am the Vine, and you are the branches.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)