16 “And a man shall say, If therefore there is fruit to a just man”. Behold, before that there comes that which is promised, before that there is given life everlasting, before that ungodly men are cast forth into fire everlasting, here in this life there is fruit to the just man. What fruit? “In hope rejoicing, in tribulation enduring.” What fruit to the just man? “We glory in tribulations, knowing that tribulation works patience, but patience probation, but probation hope: but hope confounds not: because the love of God is shed abroad in our hearts through the Holy Spirit, that has been given to us.” Does he rejoice that is a drunkard; and does he not rejoice that is just?
In love there is fruit to a just man. Miserable the one, even when he makes himself drunken: blessed the other, even when he hungers and thirsts. The one wine-bibbing does gorge, the other hope does feed. Let him see therefore the punishment of the other, his own rejoicing, and let him think of God. He that has given even now such joy of faith, of hope, of charity, of the truth of His Scriptures, what manner of joy is He making ready against the end? In the way thus He feeds, in his home how shall He fill him?
“And a man shall say, If therefore there is fruit to the just man.” Let them that see believe, and see, and perceive. Rejoice shall the just man when he shall have seen vengeance. But if he has not eyes whence he may see vengeance, he will be made sad, and will not be amended by it. But if he sees it, he sees what difference there is between the darkened eye of the heart, and the eye enlightened of the heart: between the coolness of chastity and the flame of lust, between the security of hope and the fear there is in crime.
When he shall have seen this, let him separate himself, and wash his hands in the blood of the same. Let him profit by the comparison, and say, “Therefore there is fruit to the just man: therefore there is a God judging them in the earth.” Not yet in that life, not yet in fire eternal, not yet in the lower places, but here in earth....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)