13 The grass withers, the flower of sinners dies away: what of the righteous? “The righteous shall flourish like a palm tree”. The ungodly are green as grass; “The righteous shall flourish like a palm tree.” By the palm tree he signifies height. Possibly he had also this meaning in the palm, that in its extremities it is beautiful: so that you may trace its beginning from the earth, its end in its topmost branches, wherein its whole beauty dwells. The rough root appears in the earth, the beautiful foliage toward the sky. Your beauty too, then, shall be in the end. Your root is fixed fast: but our root is upward. For our root is Christ, who has ascended into heaven. Humbled, he shall be exalted; “he shall spread abroad like a cedar in Libanus.” See what trees he spoke of: the righteous shall flourish like a palm-tree: and shall spread abroad like a cedar in Libanus. When the sun has gone forth, does the palm-tree wither? Does the cedar die? But when the sun has been glowing for some hours, the grass dries up. The judgment, therefore, shall come, that sinners may wither, and the faithful flourish.
14. “Such as are planted in the house of the Lord, shall flourish in the courts of the house of our God”. “They shall be yet more increased in fruitful old age, and shall be quiet, that they may show it forth”. Such is the Sabbath, which but a little while ago I commended unto you, whence the Psalm has its title. “They shall be quiet, that they may show it forth.” Wherefore are they quiet that show it forth? The grass of sinners moves them not: the cedar and palm-tree not even in tempests are bent. They are therefore quiet, that they may show it forth: and with reason, since at present they must show it forth even unto men who mock at it. O wretched men, who are lovers of the world! Those who are planted in the house of the Lord, show it to you: those who praise the Lord with song and lute, in word and deed, show it forth to you, and tell you. Be not seduced by the prosperity of the wicked, admire not the flower of grass: admire not those who are happy only for a season, but miserable unto eternity....If you wish to flourish like a palm-tree, and to spread abroad like a cedar in Libanus, and not to wither like grass when the sun is hot; as those who appear to flourish when the sun is absent. If then ye wish not to be as grass, but as the palm-tree and the cedar, what will you show forth? “How true the Lord my strength is: and that there is no unrighteousness in Him.” How is it there is no unrighteousness? A man commits so great crimes; he is well, he has sons, a plentiful house, he is full of pride, is exalted by his honours, is revenged on his enemies, and does every evil deed; another man, innocent, attending to his own affairs, not robbing another's goods, doing nothing against any one, suffers in chains, in prison, tosses and sighs in poverty. How is it that there is no unrighteousness in Him? Be quiet, and you shall know: for you are disturbed, and in your chamber thou dost darken your light. The eternal God does wish to shine upon you: do not then make you cloudy weather from your own disturbed mind. Be quiet within yourself, and see what I say unto you. Because God is eternal, because for the present He spares the bad, bringing them to repentance: He scourges the good, instructing them in the way unto the kingdom of heaven: “There is no unrighteousness in Him:” fear not....What, if He leaves this man unpunished now, because he is doomed to hear, “Depart into everlasting fire.” But when? When you shall be placed at the right hand, then shall it be said to those placed on the left, “Depart into the everlasting fire, which is prepared for the devil and his angels.” Let not therefore those things move you: Be quiet, keep Sabbath, and show “how true the Lord my strength is: and that there is no unrighteousness in Him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)