12....“Who covers the heaven with clouds, who prepares rain for the earth”. Now you are alarmed, because you can not see the heaven: when it has rained you shall gather fruit, and shall see clear sky. Perhaps our God has done this. For had we not the obscurity of Scripture as an occasion, we should not say to you those things wherein ye rejoice. This then perhaps is the rain whereat ye rejoice. It would not be possible for it to be expressed to you by our tongue, were it not that God covers with clouds of figures the heaven of the Scriptures. For this purpose willed He that the words of the Prophets should be obscure, that the servants of God might afterwards have that by interpreting which they might flow over the ears and hearts of men, that they might receive from the clouds of God the fatness of spiritual joy. “Who makes grass to grow upon the mountains, and herb for the service of men.” Behold the fruit of the rain. “Who makes,” says he, “grass to grow upon the mountains.” Does it not also grow upon the low ground? Yes, but it is a great thing that it grows “on the mountains.”...For nothing could be more barren than the hard mountains. “And herb for the service of men.” What “service”? Listen to Paul himself. “And ourselves,” says he, “your servants for Jesus Christ's sake.” He who said, “If we have sown unto you spiritual things, is it a great thing if we reap your carnal things?” yet said, that he was a “servant.” For we are your servants, brethren. Let none of us speak of himself, as though he were greater than you. We shall be greater if we are more humble. “But whosoever will be great among you” (it is the Lord's saying), “shall be your servant.” Paul the Apostle, indeed, living by his own labour, refused even to receive “the grass of the mountains;” he chose to want; nevertheless, the mountains gave “grass.” Because he chose not to receive, ought the mountains therefore not to give, and so to remain barren? Fruit is due to the rain, food is due to the servant, as the Lord says, “Eat such things as they give you:” and that they should not think that they gave anything of their own, He added, “for the labourer is worthy of his hire.”
13....Just now has been read, “Give to every one that asks of you;” and in another place Scripture says, “Let alms sweat in your hand, till you find a righteous man to whom to give it.” One there is who seeks you, another you ought to seek. Leave not indeed him who seeks you empty, for, “give to every one that asks of you;” yet still there is another whom you ought to seek; “find a righteous man to whom to give it.” You will never do this, unless you have somewhat set aside from your substance, each what pleases him according to the needs of his family, as a sort of debt to be paid to the treasury. If Christ have not a state of His own, neither has He a treasury....Cut off then and prune off some fixed sum either from your yearly profits or your daily gains, else you seem as it were to give of your capital, and your hand must needs hesitate, when you put it forth to that which you have not vowed. Cut off some part of your income; a tenth if you choose, though that is but little. For it is said that the Pharisees gave a tenth; “I fast twice in the week, I give tithes of all that I possess.” And what says the Lord? “Unless your righteousness exceeds the righteousness of the Scribes and Pharisees, you shall not enter into the kingdom of heaven.” He whose righteousness you ought to exceed, gives a tenth: you give not even a thousandth. How will you surpass him whom you match not? “Who prepares rain for the earth.”
14. “And gives unto the cattle their food”. These are the cattle he means, even God's flocks. God defrauds not His flock of their food through men, for whose “service He makes the grass to grow.” “And to the young of the ravens that call upon Him.” Shall we perchance think this, that the ravens call upon God to give them their food? Think not that the unreasoning creature calls upon God: no creature knows how to call upon God, save the reasonable alone. Consider it as spoken in a figure, lest you think, as some evil men say, that the souls of men migrate into cattle, dogs, swine, ravens. Give this no place in your hearts or in your faith. The soul of man is made after the image of God: He will not give His image to dog or swine. Who are “the young of the ravens”? The Israelites used to say that they alone were righteous, because to them the Law had been given: all other men of every nation they used to call sinners. And in truth all nations were given up to sin, to idolatry, to the worship of stones and stocks: but did they continue so? Although the ravens themselves, our fathers, did not, yet we, “the young of the ravens,” do call upon God....For “the young of the ravens,” who seemed to worship the images of their forefathers, have advanced, and turned to God. And now you hear “the young of the ravens” calling upon the one God. What then? Do you say to “the young of the ravens,” “have you left your father?” Plainly I have, says he; for he is a raven who calls not upon God. I, “the young of the raven,” do call upon God.
15. “In the power of an horse He will not take pleasure”. The power “of an horse” is pride. For the horse seems adapted as it were to bear a man aloft, that he may be more uplifted as he goes. And in truth he has a neck which typifies a sort of pride. Let not men exalt themselves upon their worth, let them not think themselves uplifted by their distinctions; let them beware lest they be thrown by an untamed horse....“Nor in the tabernacle of a man will He delight.” For the tabernacle of the Lord is the Holy Church spread throughout the whole world. Heretics, separating themselves from the Church's tabernacles, have set up tabernacles for themselves. For if perchance it be the lot of any, who is good and pious, who confesses his own weakness, who is “the young of a raven that calls on God,” not to enjoy worldly distinction, he goes not out of the Church, he sets not up for himself a tent outside the Church, wherein God will not delight. But what says he? “I have chosen to be cast away in the house of God, rather than to dwell in the tents of sinners.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)