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.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)