15 As yet the punishments of the lower places have not come, as yet fire everlasting has not come: let him that is growing in God compare himself now with an ungodly man, a blind heart with an enlightened heart: compare ye two men, one seeing and one not seeing in the flesh. And what so great thing is vision of the flesh? Did Tobias by any means have fleshly eyes? His own son had, and he had not; and the way of life a blind man to one seeing did show. Therefore when you see that punishment, rejoice, because in it you are not.
Therefore says the Scripture, “The just man shall rejoice when he shall have seen vengeance”. Not that future punishment; for see what follows: “his hands he shall wash in the blood of the sinner.” What is this? Let your love attend. When man-slayers are smitten, ought anywise innocent men to go there and wash their hands? But what is, “in the blood of the sinner he shall wash his hands”? When a just man sees the punishment of a sinner, he grows himself; and the death of one is the life of another. For if spiritually blood runs from those that within are dead, do thou, seeing such vengeance, wash therein your hands; for the future more cleanly live. And how shall he wash his hands, if a just man he is? For what has he on his hands to be washed, if just he is? “But the just man of faith shall live.” Just men therefore he has called believers: and from the time that you have believed, at once you begin to be called just. For there has been made a remission of sins. Even if out of that remaining part of your life some sins are yours, which cannot but flow in, like water from the sea into the hold; nevertheless, because you have believed, when you shall have seen him that altogether is turned away from God to be slain in that blindness, there falling upon him that fire so that he see not the sun— then do thou that now through faith see Christ, in order that you may see in substance (because the just man lives of faith), observe the ungodly man dying, and purge yourself from sins. So you shall wash in a manner your hands in the blood of the sinner.
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)