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.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)