16 “Your own is the day and Your own is the night”. Who is ignorant of this, seeing that He has Himself made all these things; for by the Word were made all things? To that very One Himself who has wrought Salvation in the midst of the earth, to Him is said, “Your own is the night.” Something here we ought to perceive which belongs to that very Salvation which He has wrought in the midst of the earth. “Your own is the day.” Who are these? The spiritual. “And Your own is the night.” Who are these? The carnal....“You have made perfect sun and moon:” the sun, spiritual men, the moon, carnal men. As yet carnal he is, may he not be forsaken, and may he too be made perfect. The sun, as it were a wise man: the moon, as it were an unwise man: You have not however forsaken. For thus it is written, “A wise man endures as the sun, but a foolish man as the moon is changed.” What then? Because the sun endures, that is, because the wise man endures as the sun, a foolish man is changed like the moon, is one as yet carnal, as yet unwise, to be forsaken? And where is that which has been said by the Apostle, “To the wise and unwise a debtor I am”?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)