16 But what adds he? “The Lord will delight in them that fear Him, and in them that hope in His mercy”. A robber is feared, and a wild beast is feared, and an unjust and powerful man is much feared. “The Lord will delight in them that hope in His mercy.” Behold, Judas, who betrayed our Lord, feared, but he did not hope in His mercy....It is well indeed that you have feared, but only if you trusted in His mercy, whom you have feared. He in despair “went and hanged himself.” In such wise then fear the Lord, that you trust in His mercy....
17. “Praise in unison, O Jerusalem, your God”. Abiding yet in captivity, they behold those flocks, or rather, the one flock of all its citizens, gathered from all sides into that city; they see the joy of the mass, now after threshings and winnowings placed in the garner, fearing nothing, suffering no toil nor trouble; and, as yet abiding here, in the midst of the threshing they send forward their joy of hope, and pant for it, joining as it were their hearts to the Angels of God, and to that people which shall abide with them in joy for ever. For what will you then do, O Jerusalem? Surely toil and groaning will pass away. What will you do? Will you plough, or sow, or plant vines, or make voyages, or trade? What will you do? Will it still be your duty to be engaged in the works thou now doest, good though they are, and spring from mercy? Consider your numbers, consider on all sides your company: see whether any hungers, for you to give bread to; see whether any thirsts, for you to give a cup of cold water to; see whether any is a stranger, for you to take in; see whether any is sick, for you to visit; see whether any is at strife, for you to reconcile him; see whether any is dying, for you to bury him. What then will you do? “Praise in unison, O Jerusalem, your God.” Behold, this is your business. As is wont to be said in inscriptions, “Use it and be happy.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)