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”?
17. “You have made all the ends of the earth”....Behold in what manner He has made the ends of the earth, that has wrought Salvation in the midst of the earth. “You have made all the ends of the earth. Summer and spring You have made them.” Men fervent in the Spirit are the summer. You, I say, hast made men fervent in the Spirit: You have made also the novices in the Faith, they are the “spring.” “Summer and Spring You have made them.” They shall not glory as if they have not received: “You have made them.”
18. “Be mindful of this Your creature”. Of what creature of Yours? “The enemy has reviled the Lord.” O Asaph, grieve over your old blindness in understanding: “the enemy has reviled the Lord.” It was said to Christ in His own nation, “a sinner is this Man: we know not whence He is:” we know Moses, to him spoke God; this Man is a Samaritan. “And the unwise people has provoked Your name.” The unwise people Asaph was at that time, but not the understanding of Asaph at that time. What is said in the former Psalm? “As it were a beast I have become unto You, and I am always with You:” because He went not to the gods and idols of the Gentiles. Although he knew not, being like a beast, yet he knew again as a man. For he said, “alway I am with You, like a beast:” and what afterwards in that place in the same Psalm, where Asaph is? “You have held the hand of my right hand, in Your will You have conducted me, and with glory You have taken me up.” In Your will, not in my righteousness: by Your gift, not by my work. Therefore here also, “the enemy has reviled the Lord: and the unwise people has provoked Your name.” Have they all then perished? Far be it....For even the Apostle Paul through unbelief had been broken, and through faith unto the root he was restored. So evidently “the unwise people provoked Your name,” when it was said, “If Son of God He is let Him come down from the Cross.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)