23 I will not weaken the evidence for this truth by intermixing words of my own. Let us hear the Father, when the baptism of Jesus Christ was accomplished, speaking, as often, concerning His Only-begotten, in order to save us from being misled by His visible body into a failure to recognise Him as the Son. His words are:— This is My beloved Son, in Whom I am well pleased. Is the truth presented here with dim outlines? Is the proclamation made in uncertain tones?
The promise of the Virgin birth brought by the angel from the Holy Ghost, the guiding star of the Magi, the reverence paid Him in His cradle, the majesty, attested by the Baptist, of Him Who condescended to be baptized; all these are deemed an insufficient witness to His glory. The Father Himself speaks from heaven, and His words are, This is My Son. What means this evidence, not of titles, but of pronouns? Titles may be appended to names at will; pronouns are a sure indication of the persons to whom they refer.
And here we have, in This and My, the clearest of indications. Mark the true meaning and the purpose of the words. You have read, I have begotten sons, and have raised them up; but you did not read there My sons, for He had begotten Himself those sons by division among the Gentiles, and from the people of His inheritance. And lest we should suppose that the name Son was given as an additional title to God the Only-begotten, to signify His share by adoption in some joint heritage, His true nature is expressed by the pronoun which gives the indubitable sense of ownership.
I will allow you to interpret the word Son, if you will, as signifying that Christ is one of a number, if you can furnish an instance where it is said of another of that number, This is My Son. If, on the other hand, This is My Son be His peculiar designation, why accuse the Father, when He asserts His ownership, of making an unfounded claim? When He says This is My Son, may we not paraphrase His meaning thus:— “He has given to others the title of sons, but He Himself is My own Son; I have given the name to multitudes by adoption, but this Son is My very own.
Seek not for another lest you lose your faith that This is He. By gesture and by voice, by This, and My, and Son, I declare Him to you.” And now what reasonable excuse remains for lack of faith? This, and nothing less than this, it was that the Father's voice proclaimed. He willed that we should not be left in ignorance of the nature of Him Who came to be baptized, that He might fulfil all righteousness; that by the voice of God we might recognise as the Son of God Him Who was visible as Man, to accomplish the mystery of our salvation.
Source: On the Trinity (New Advent)