10 And considering the length of the way, what said he to himself? “And I said, Peradventure the darkness shall overwhelm me”11. Lo, now I have believed in Christ, now am I wafted aloft on the wings of twofold love....Regarding the length of the way, I said to myself, “And the night was light in my delight.” The night was made to me light, because in the night I despaired of being able to cross so great a sea, to surmount so long a journey, to reach the utmost parts by persevering to the end. Thanks to Him who sought me when a runaway, who smote my back with strokes of the scourge, who by calling me recalled me from destruction, who made my night light. For it is night so long as we are passing through this life. How was the night made light? Because Christ came down into the night....
11. “For darkness shall not be darkened by You”. Do not thou then darken your darkness; God darkens it not, but enlightens it yet more; for to Him is said in another Psalm, “You, Lord, shall light my candle: my God shall enlighten my darkness.” But who are they who “darken their darkness,” which God darkens not? Evil men, perverse men; when they sin, verily they are darkness; when they confess not their sins which they have committed but go on to defend them, they “darken their darkness.” Wherefore now if you have sinned you are in darkness, but by confessing your darkness you shall obtain to have your darkness lightened; by defending your darkness, you shall “darken your darkness.” And where will you escape from double darkness, who wast in difficulty in single darkness?...Let us not “darken our darkness” by defending our sins, and “the night shall be light in our delight.”
12. “And night shall be lightened as the day.” “Night, as the day.” “Day” to us is worldly prosperity, night adversity in this world: but, if we learn that it is by the desert of our sins that we suffer adversities, and our Father's scourges are sweet to us, that the Judge's sentence may not be bitter to us, so shall we find the darkness of this night to be, as it were, the light of this night....But when Christ our Lord has come, and has dwelt in the soul by faith, and promised other light, and inspired and given patience, and warned a man not to delight in prosperity or to be crushed by adversity, the man, being faithful, begins to treat this world with indifference; not to be lifted up when prosperity befalls him, nor crushed when adversity, but in all things to praise God, not only when he abounds, but also when he loses; not only when he is in health, but also when he is sick....“As is His darkness, so is also His light.” His darkness overwhelms me not, because His light lifts me not up.
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.
14. “I will confess to You, O Lord, for terribly have You been made wonderful: wondrous are Your works, and my soul knows it right well”. Aforetime “Your knowledge was made wonderful from me, it had waxed great, nor could I attain unto it.” From me then “it had waxed great.” Whence does “my soul” now “know right well,” save because the “night is light in my delight?” save because Your grace has come unto me, and enlightened my darkness? save because You have possessed my reins? save because You have taken me up from my mother's womb?
15. “My bone is not hid from You, which You have made in secret”. “His bone,” he says. What the people call ossum, is in Latin called os. This is the word in the Greek. For we might think the word os is here the one which makes in the plural ora, not os (short), which makes ossa. He says then, I have a certain bone (ossum) in secret. For this word let us prefer to use; better is it that scholars find fault with us, than that the people understand us not. “There is then,” says he, a certain bone of mine, within, hidden; You have made within a bone for me in secret, yet is it not hidden from You. In secret have You made it, but have You therefore hidden it from Yourself? This my bone made by You in secret men see not, men know not: You know, who hast made. What “bone” then means he, brethren? Let us seek it, it is “in secret.” But because as Christians we are speaking in the Name of the Lord to Christians, now we find what bone is of this kind. It is a sort of inward strength; for strength and fortitude are understood to be in the bones. There is then a sort of inward strength of the soul, wherein it is not broken. Whatever tortures, whatever tribulations, whatever adversities rage around, that which God has made strong in secret in us, cannot be broken, yields not. For by God is made a certain strength of patience, of which is said in another Psalm, “But my soul shall be subjected to God, for of Him is my patience.”...Wherein do you glory? “In tribulations, knowing that tribulation works patience.” See how that strength is fashioned within in his heart: “because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.” So is fashioned and made strong that hidden bone, that it makes us even to glory in tribulations. But to men we seem wretched, because that which we have within is hidden from them. “And my substance is in the lower parts of the earth.” Behold, in flesh is my substance, yet have I a bone within, which You have fashioned, such as to cause me never to yield to any persecutions of this lower region, where still my substance is. For what great matter is it, if an Angel be brave? This is a great matter, if flesh is brave. And whence is flesh brave, whence is an earthen vessel brave, save because in it is made a bone in secret?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)