3 And again he says, “both all you earthborn, and sons of men”. The expression “earthborn” refers to sinners; the expression “sons of men” to the faithful and righteous. You see then that this distinction is observed. Who are the “earthborn”? The children of the earth. Who are the children of the earth? They who desire earthly inheritances. Who are the “sons of men”? They who appertain to the Son of Man. We have already before explained this distinction to your Sanctity, and have concluded that Adam was a man, but not the son of man; that Christ was the Son of Man, but was God also.
For whosoever pertain to Adam, are “earthborn:” whosoever pertain to Christ, are “sons of men.” Nevertheless, let all hear, I withhold my discourse from no one. If one is “earthborn,” let him hear, because of the judgment: another is a “son of man,” let him hear for the kingdom's sake. “The rich and poor together.” Again, the same words are repeated. The expression “rich” refers to the “earthborn;” but the word “poor” to the “sons of men.” By the “rich” understand the proud, by the “poor” the humble....He says in another Psalm, “The poor shall eat and be satisfied.” How has he commended the poor?
“The poor shall eat and be satisfied.” What eat they? That Food which the faithful know. How shall they be satisfied? By imitating the Passion of their Lord, and not without cause receiving their recompense. “The poor shall eat and be satisfied, and they shall praise the Lord who seek Him.” What of the rich? Even they eat. But how eat they? “All the rich upon the earth have eaten and worshipped.” He said not, “Have eaten and are satisfied;” but, “have eaten and worshipped.” They worship God indeed, but they will not display brotherly humaneness.
These eat and worship; those eat and are filled: yet both eat. Of the eater what he eats is required: let him not be forbidden by the distributor to eat, but let him be admonished to fear him who does require his account. Let these words then be heard by sinners and righteous, nations, and those who inhabit the world, “earthborn and sons of men, the rich and the poor together:” not divided, not separated. That is for the time of the harvest to do, the hand of the winnower will effect that. Now together let rich and poor hear, let goats and sheep feed in the same pasture, until He come who shall separate the one on His right hand, the other on His left. Let them all hear together the teacher, lest separated from one another they hear the voice of the Judge.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)