I answer that, As stated above (Q. 5, A. 1), it was unbecoming that the Son of God, who is the Truth of the Father, should make use of anything unreal; wherefore He took, not an imaginary, but a real body. And since the Holy Ghost is called the Spirit of Truth, as appears from John 16:13, therefore He too made a real dove in which to appear, though He did not assume it into unity of person. Wherefore, after the words quoted above, Augustine adds: "Just as it behooved the Son of God not to deceive men, so it behooved the Holy Ghost not to deceive. But it was easy for Almighty God, who created all creatures out of nothing, to frame the body of a real dove without the help of other doves, just as it was easy for Him to form a true body in Mary's womb without the seed of a man: since the corporeal creature obeys its Lord's command and will, both in the mother's womb in forming a man, and in the world itself in forming a dove."
Reply Obj. 1: The Holy Ghost is said to have descended in the shape or semblance of a dove, not in the sense that the dove was not real, but in order to show that He did not appear in the form of His substance.
Reply Obj. 2: It was not superfluous to form a real dove, in which the Holy Ghost might appear, because by the very reality of the dove the reality of the Holy Ghost and of His effects is signified.
Reply Obj. 3: The properties of the dove lead us to understand the dove's nature and the effects of the Holy Ghost in the same way. Because from the very fact that the dove has such properties, it results that it signifies the Holy Ghost. _______________________
EIGHTH ARTICLE [III, Q. 39, Art. 8]
Whether It Was Becoming, When Christ Was Baptized That the Father's
Voice Should Be Heard, Bearing Witness to the Son?
Objection 1: It would seem that it was unbecoming when Christ was baptized for the Father's voice to be heard bearing witness to the Son. For the Son and the Holy Ghost, according as they have appeared visibly, are said to have been visibly sent. But it does not become the Father to be sent, as Augustine makes it clear (De Trin. ii). Neither, therefore, (does it become Him) to appear.
Obj. 2: Further, the voice gives expression to the word conceived in the heart. But the Father is not the Word. Therefore He is unfittingly manifested by a voice.
Obj. 3: Further, the Man-Christ did not begin to be Son of God at His baptism, as some heretics have stated: but He was the Son of God from the beginning of His conception. Therefore the Father's voice should have proclaimed Christ's Godhead at His nativity rather than at His baptism.
On the contrary, It is written (Matt. 3:17): "Behold a voice from heaven, saying: This is My beloved Son in whom I am well pleased."