15 But perhaps you will say, I lose this light. “There is sprung up a light for the righteous”. What light do you fear you may lose? Do you fear you may be in darkness? Fear not you may lose light; nay, fear lest while you are guarding against the loss of this light, you may lose that true light. For we see to whom that light is given which you fear losing, and with whom it is shared. Do the righteous only see this sun, when He makes it rise over the just and unjust, and rains upon the just and unjust? Wicked men, robbers, the unchaste, beasts, flies, worms, see that light together with you. What sort of light does He keep for the righteous, who gives this even to such as these? Deservedly the Martyrs beheld this light in faith; for they who despised this light of the sun, had some light in their eyes, which they longed for, who rejected this. Do you imagine that they were really in misery, when they walked in chains? Spacious was the prison to the faithful, light were the chains to the confessors. They who preached Christ amid their torments, had joy in the iron-chair. What light has sprung up for the righteous? Not that which springs up for the unrighteous; not that which He causes to rise over the good and bad. There is a different light which springs up to the righteous; of which light, that never rose upon themselves, the unrighteous shall in the end say, “Therefore have we erred from the way of truth, and the light of righteousness has not shined upon us, and the sun of righteousness rose not upon us.” Behold, by loving this sun they have lain in the darkness of the heart. What did it profit them to have seen with their eyes this sun, and not in mind to have seen that light? Tobit was blind, but he used to teach his son the way of God. You know this, that Tobit warned his son, and said to him, “Son, give alms of your substance; because that alms suffer not to come into darkness.” Even he who was in darkness spoke thus....Do you wish to know that light? Be true-hearted. What is, be true-hearted? Be not of a crooked heart before God, withstanding His will, and wishing to bend Him unto you, and not to rule yourself to please Him; and you will feel the joyful gladness which all the true-hearted know.
16. “Be glad, you righteous”. Perhaps already the faithful hearing the word, “Be glad,” are thinking of banquets, preparing cups, waiting for the season of roses; because it is said, “Be glad, you righteous!” See what follows, “Be glad in the Lord.” You are waiting for the season of spring, that you may be glad: you have the Lord for joyful gladness, the Lord is always with you, He has no special season; you have Him by night, you have Him by day. Be true-hearted; and you have ever joy from Him. For that joy which is after the fashion of the world, is not true joy. Hear the prophet Isaiah: “There is no joy, says my God, to the wicked.” What the wicked call joy is not joy, such as he knew who made no account of their joy: let us believe him, brethren. He was a man, but he knew both kinds of joy. He certainly knew the joys of the cup, for he was a man, he knew the joy of the table, he knew the joys of marriage, he knew those joys worldly and luxurious. He who knew them says with confidence, “There is no joy to the wicked, says the Lord.” But it is not man who speaks, it is the Lord....But you say, I see not that light which Isaiah saw. Believe, and you shall see it. For perhaps you have not the eye to see it; for it is an eye by which that beauty is discerned. For as there is an eye of the flesh, by means of which this light is seen: so there is an eye of the heart, by which that joy is perceived: perhaps that eye is wounded, dimmed, disturbed by passion, by avarice, by indulgence, by senseless lust; your eye is disturbed: you can not see that light. Believe, before you see; you shall be healed, and shall see.
17. “And confess to the remembrance of His holiness.” Now made glad, now rejoicing in the Lord, confess unto Him; for unless it were His will, you would not rejoice in Him. For the Lord Himself says: “These things I have spoken to you: that in Me ye might have peace. But in the world you shall have tribulation.” If you are Christians, look for tribulations in this world; look not for more peaceful and better times. Brethren, you deceive yourselves; what the Gospel does not promise you, promise not to yourselves. You know what the Gospel says; we are speaking to Christians; we ought not to disobey the faith. The Gospel says this, that in the last times many evils, many stumbling-blocks, many tribulations, much iniquity, shall abound; but he that shall endure unto the end, the same shall be saved. “The love,” it says, “of many shall wax cold.” Whosoever then has been steadfastly fervent in spirit, as the Apostle says, “fervent in spirit,” his love shall not wax cold: because “the love of God is shed abroad in our hearts by the Holy Ghost, which is given unto us.” Let no man therefore promise himself what the Gospel does not promise. Behold, happier times will come, and I am doing this, and purchasing this. It is good for you to listen to Him who is not deceived, nor has deceived any man, who promised you joy not here, but in Himself; and when all here has passed away, to hope that with Him you will for ever reign; lest when thou dost wish to reign here, you may neither enjoy gladness here, nor find it there.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)