1 The inscription of this Psalm is, “To the end for the hidden things of the Son, a Psalm of David himself.” As to the hidden things of the Son there may be a question: but since he has not added whose, the very only-begotten Son of God should be understood. For where a Psalm has been inscribed of the son of David, “When,” he says, “he fled from the face of Absalom his son;” although his name even was mentioned, and therefore there could be no obscurity as to whom it was spoken of: yet it is not merely said, from the face of son Absalom; but “his” is added. But here both because “his” is not added, and much is said of the Gentiles, it cannot properly be taken of Absalom. For the war which that abandoned one waged with his father, no way relates to the Gentiles, since there the people of Israel only were divided against themselves. This Psalm is then sung for the hidden things of the only-begotten Son of God. For the Lord Himself too, when, without addition, He uses the word Son, would have Himself, the Only-begotten to be understood; as where He says, “If the Son shall make you free, then shall you be free indeed.” For He said not, the Son of God; but in saying merely, Son, He gives us to understand whose Son it is. Which form of expression nothing admits of, save His excellency of whom we so speak, that, though we name Him not, He can be understood. For so we say, it rains, clears up, thunders, and such like expressions; and we do not add who does it all; for that the excellency of the doer spontaneously presents itself to all men's minds, and does not want words. What then are the hidden things of the Son? By which expression we must first understand that there are some things of the Son manifest, from which those are distinguished which are called hidden. Wherefore since we believe two advents of the Lord, one past, which the Jews understood not: the other future, which we both hope for; and since the one which the Jews understood not, profited the Gentiles; “For the hidden things of the Son” is not unsuitably understood to be spoken of this advent, in which “blindness in part is happened to Israel, that the fullness of the Gentiles might come in.”
For notice of two judgments is conveyed to us throughout the Scriptures, if any one will give heed to them, one hidden, the other manifest. The hidden one is passing now, of which the Apostle Peter says, “The time has come that judgment should begin from the house of the Lord.” The hidden judgment accordingly is the pain, by which now each man is either exercised to purification, or warned to conversion, or if he despise the calling and discipline of God, is blinded unto damnation. But the manifest judgment is that in which the Lord, at His coming, will judge the quick and the dead, all men confessing that it is He by whom both rewards shall be assigned to the good, and punishments to the evil. But then that confession will avail, not to the remedy of evils, but to the accumulation of damnation. Of these two judgments, the one hidden, the other manifest, the Lord seems to me to have spoken, where He says, “Whoso believes in Me has passed from death unto life, and shall not come into judgment;” into the manifest judgment, that is. For that which passes from death unto life by means of some affliction, whereby “He scourges every son whom He receives,” is the hidden judgment. “But whoso believes not,” says He, “has been judged already:” that is, by this hidden judgment has been already prepared for that manifest one. These two judgments we read of also in Wisdom, whence it is written, “Therefore unto them, as to children without the use of reason, You gave a judgment to mock them; But they that have not been corrected by this judgment have felt a judgment worthy of God.” Whoso then are not corrected by this hidden judgment of God, shall most worthily be punished by that manifest one....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)