Augustine of Hippo, on Ps 145:3

The poor Christian sees this, ignoble, sighing in daily labor with groans, and perhaps says to himself: "What good is it to me that I have become a Christian? Am I better off than he who is not, than he who does not believe in Christ, than he who blasphemes my God?" The psalm advises him: do not put your trust in princes. Why does the flower of the grass delight you? All flesh is grass, as the Prophet says; he not only speaks but exclaims. The Lord exclaimed to him: Cry out, He said. And he answered: What should I cry out? All flesh is grass, and all the glory of the flesh is like the flower of the grass. The grass withers, the flower falls. Is everything lost then? By no means. But the word of the Lord endures forever. Why does the grass delight you? Behold, the grass has perished. Do you want not to perish? Hold on to the Word. So too in this psalm. For the poor, humble Christian might be looking at a rich and powerful pagan, looking at the flower of the grass, and perhaps choosing him as a patron rather than God. The psalm addresses him: Do not trust in princes, in the sons of men in whom there is no salvation. He immediately responds: "Does he say this about one who has salvation? Behold, he is healthy. Today I see him vigorous. I myself am constantly sick with misery." Why do you look at things that please and delight you? There is no salvation. His spirit will depart, and he will return to his earth. Behold all his salvation, a vapor appearing for a moment. His spirit will depart, and he will return to his earth. Let a few years pass. Let the river move, as usually happens, running through various tombs of the dead. Distinguish the bones of the rich man from the bones of the poor man. Upon his spirit departing, he returns to his earth. He said nothing of his spirit for the best because when he lived, he thought nothing spiritually. He will return to his earth, according to the flesh, surely, according to the body, where he flourished, where he was carried away, where he deceived you, in the felicity of whose flesh he misled you. His spirit will depart, and he will return to his earth; on that day all their thoughts will perish. Those thoughts which were earthly: "Behold, I do this, behold, I accomplish, behold, I reach, behold, I buy that, behold, I acquire that, behold, I reach such and such honor," on that day all his thoughts will perish. But if, because the word of the Lord endures forever, you hold to the Word to give you eternal life, not only will your thought not perish then, but it will come then. When his perishes, yours will come. For he thought about temporal and earthly things, adding wealth to wealth, augmenting monetary resources, striving for honors, swelling with power. Therefore, because he thought such things, on that day all his thoughts will perish. But you, if you thought, because you have become a Christian, not about temporal felicity, but eternal rest when your body returns to its earth, then your soul finds its rest.