21 In your reflections therefore on yourself, think of your low estate, think of your dust: be not lifted up: if you are anything better, you will be so by His Grace, you will be so by His mercy. For hear what follows: “but the mercy of the Lord endures for ever and ever upon them that fear Him”. You who fear not Him, will be grass, and in grass, and in torment with the grass: for the flesh shall arise unto the torment. Let those who fear Him rejoice, because His mercy is upon them.
22. “And His righteousness upon children's children”. He speaks of reward, “upon children's children.” How many servants of God are there who have not children, how much less children's children? But He calls our works our children; the reward of works, our “children's children.” “Even upon such as keep His covenant.” Let men beware that all may not conceive what is here said to belong to themselves: let them choose, while they have the choice. “And keep in memory His commandments to do them.” You were already disposed to flatter yourself, and perhaps to recite to me the Psalter, which I have not by heart, or from memory to say over the whole Law. Clearly you are better in point of memory than I, better than any righteous man who does not know the Law word for word: but see that thou keep the commandments. But how should you keep them? Not by memory, but by life. “Such as keep in memory His commandments:” not, to recite them; but, “to do them.” And now perhaps each man's soul is disturbed. Who remembers all the commandments of God? Who remembers all the writings of God? Lo, I wish not only to hold them in my memory, but also to do them in my works: but who remembers them all? Fear not: He burdens you not: “on two commandments hang all the Law and the Prophets.”...
23. “The Lord has prepared His throne in heaven”. Who but Christ has prepared His throne in heaven? He who descended and ascended, He who died, and rose from the dead, He who lifted up to heaven the manhood He had assumed, has Himself prepared His throne in heaven. The throne is the seat of the Judge: observe therefore ye who hear, that “He has prepared His throne in heaven.”...The kingdom is the Lord's, and He shall be the Governor among the people. “And His kingdom shall rule over all.”
24. “Bless ye the Lord, you Angels of His, you that are mighty in strength: ye that fulfil His word”. By the word of God, then, you are not righteous, nor faithful, unless when thou dost it. “You that are mighty in strength, you that fulfil His commandment, and hearken unto the voice of His words.”
25. “Bless ye the Lord, all you His hosts: ye servants of His that do His pleasure”. All you angels, all you that are mighty in strength: ye that do His word: all you His hosts, you servants of His that do His pleasure, do ye, you bless the Lord. For all they who live wickedly, though their tongues be silent, by their lips do curse the Lord. What does it profit if your tongue sings a hymn, while your life breathes sacrilege? By living ill you have set many tongues to blasphemy. Your tongue is given to the hymn, the tongues of those who behold you, to blasphemy. If then thou dost wish to bless the Lord, do His word, do His will....
26. “Bless ye the Lord, all you works of His, in all places of His dominion”. Therefore in every place. Let Him not be blessed where He rules not: “in all places of His dominion.” Let no man perchance say: I cannot praise the Lord in the East, because He has departed unto the West; or, I cannot praise Him in the West, because He is in the East. “For neither from the east, nor from the west, nor yet from the desert hills. And why? God is the Judge.” He is everywhere, in such wise that everywhere He may be praised: He is in such wise on every side, that we may be joyful in Him on every side: He is in such wise blessed on every side, that on every side we may live well....“In every place of His dominion: bless thou the Lord, O my soul!” The last verse is the same as the first: blessing is at the head of the Psalm, blessing at the end; from blessing we set out, to blessing let us return, in blessing let us reign.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)