13 “For Thou, O Lord, hast possessed my reins”. The Possessor is within; He occupies not only the heart, but also the reins; not only the thoughts, but also the delights: He then possesses that whence I should feel delight at any light in this world: He occupies my reins: I know not delight, save from the inward light of His Wisdom. What then? Do you not delight that your affairs are very prosperous, times fortunate to you? Do you not delight in honour, in riches, in your family? “I do not,” says he. Wherefore? Because “You have possessed my reins, O Lord; You have taken me up from my mother's womb.” While I was in my mother's womb, I did not regard with indifference the darkness of that night and the light of that night....Now, having been taken up from the womb of that our mother, we look on them with indifference, and say, “As is His darkness, so is also His light.” Neither does earthly prosperity make us happy, nor earthly adversity wretched. We must maintain righteousness, love faith, hope in God, love God, love our neighbours also. After these toils we shall have unfailing light, day without setting. Fleeting is all the light and darkness of this night.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)