15 After the hidden things then of the Son, of which, in this Psalm, many things have been said, will come the manifest things of the Son, of which a little has been now said at the end of the same Psalm. But the title is given from the former, which here occupy the larger portion. Indeed, the very day of the Lord's advent may be rightly numbered among the hidden things of the Son, although the very presence of the Lord itself will be manifest. For of that day it is said, that no man knows it, neither angels, nor powers, nor the Son of man. What then so hidden, as that which is said to be hidden even to the Judge Himself, not as regards knowledge, but disclosure?
But concerning the hidden things of the Son, even if any one would not wish to understand the Son of God, but of David himself, to whose name the whole Psalter is attributed, for the Psalms we know are called the Psalms of David, let him give ear to those words in which it is said to the Lord, “Have mercy on us, O Son of David:” and so even in this manner let him understand the same Lord Christ, concerning whose hidden things is the inscription of this Psalm. For so likewise is it said by the Angel: “God shall give unto Him the throne of His father David.” Nor to this understanding of it is the sentence opposed in which the same Lord asks of the Jews, “If Christ be the Son of David, how then does he in spirit call Him Lord, saying, The Lord said unto my Lord, Sit on my right hand, until I put Your enemies under Your feet.” For it was said to the unskilled, who although they looked for Christ's coming, yet expected Him as man, not as the Power and Wisdom of God.
He teaches then, in that place, the most true and pure faith, that He is both the Lord of king David, in that He is the Word in the beginning, God with God, by which all things were made; and Son, in that He was made to him of the seed of David according to the flesh. For He does not say, Christ is not David's Son, but if you already hold that He is his Son, learn how He is his Lord: and do not hold in respect of Christ that He is the Son of Man, for so is He David's Son; and leave out that He is the Son of God, for so is He David's Lord.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)