4 What should you do then? “Trust in the Lord”. For they too trust, but not “in the Lord.” Their hope is perishable. Their hope is short-lived, frail, fleeting, transitory, baseless. “Trust thou in the Lord.” “Behold,” you say, “I do trust; what am I to do?”
“And do good.” Do not do that evil which you behold in those men, who are prosperous in wickedness. “Do good, and dwell in the land.” Lest haply you should be doing good without “dwelling in the land.” For it is the Church that is the Lord's land. It is her whom He, the Father, the tiller of it, waters and cultivates. For there are many that, as it were, do good works, but yet, in that they do not “dwell in the land,” they do not belong to the husbandman. Therefore do thou your good, not outside of the land, but do thou “dwell in the land.” And what shall I have?
“And you shall be fed in its riches.” What are the riches of that land? Her riches are her Lord! Her riches are her God! He it is to whom it is said, “The Lord is the portion of mine inheritance, and of my cup.” In a late discourse we suggested to you, dearly beloved, that God is our possession, and that we are at the same time God's possession. Hear how that He is Himself the riches of that land.
“Delight yourself in the Lord”. As if you had put the question, and had said “Show me the riches of that land, in which you bid me dwell,” he says, “Delight yourself in the Lord.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)