Objection 1. It would seem that the testimony of the Father's voice, saying, "This is My beloved Son," was not fittingly added; for, as it is written (Job 33:14), "God speaketh once, and repeateth not the selfsame thing the second time." But the Father's voice had testified to this at the time of (Christ's) baptism. Therefore it was not fitting that He should bear witness to it a second time.
Objection 2. Further, at the baptism the Holy Ghost appeared under the form of a dove at the same time as the Father's voice was heard. But this did not happen at the transfiguration. Therefore it seems that the testimony of the Father was made in an unfitting manner.
Objection 3. Further, Christ began to teach after His baptism. Nevertheless, the Father's voice did not then command men to hear him. Therefore neither should it have so commanded at the transfiguration.
Objection 4. Further, things should not be said to those who cannot bear them, according to John 16:12: "I have yet many things to say to you, but you cannot bear them now." But the disciples could not bear the Father's voice; for it is written (Matthew 17:6) that "the disciples hearing, fell upon their face, and were very much afraid." Therefore the Father's voice should not have been addressed to them.
On the contrary is the authority of the Gospel.
I answer that, The adoption of the sons of God is through a certain conformity of image to the natural Son of God. Now this takes place in two ways: first, by the grace of the wayfarer, which is imperfect conformity; secondly, by glory, which is perfect conformity, according to 1 John 3:2: "We are now the sons of God, and it hath not yet appeared what we shall be: we know that, when He shall appear, we shall be like to Him, because we shall see Him as He is." Since, therefore, it is in baptism that we acquire grace, while the clarity of the glory to come was foreshadowed in the transfiguration, therefore both in His baptism and in His transfiguration the natural sonship of Christ was fittingly made known by the testimony of the Father: because He alone with the Son and Holy Ghost is perfectly conscious of that perfect generation.
Reply to Objection 1. The words quoted are to be understood of God's eternal speaking, by which God the Father uttered the only-begotten and co-eternal Word. Nevertheless, it can be said that God uttered the same thing twice in a bodily voice, yet not for the same purpose, but in order to show the divers modes in which men can be partakers of the likeness of the eternal Sonship.
Reply to Objection 2. Just as in the Baptism, where the mystery of the first regeneration was proclaimed, the operation of the whole Trinity was made manifest, because the Son Incarnate was there, the Holy Ghost appeared under the form of a dove, and the Father made Himself known in the voice; so also in the transfiguration, which is the mystery of the second regeneration, the whole Trinity appears--the Father in the voice, the Son in the man, the Holy Ghost in the bright cloud; for just as in baptism He confers innocence, signified by the simplicity of the dove, so in the resurrection will He give His elect the clarity of glory and refreshment from all sorts of evil, which are signified by the bright cloud.
Reply to Objection 3. Christ came to give grace actually, and to promise glory by His words. Therefore it was fitting at the time of His transfiguration, and not at the time of His baptism, that men should be commanded to hear Him.
Reply to Objection 4. It was fitting that the disciples should be afraid and fall down on hearing the voice of the Father, to show that the glory which was then being revealed surpasses in excellence the sense and faculty of all mortal beings; according to Exodus 33:20: "Man shall not see Me and live." This is what Jerome says on Matthew 17:6: "Such is human frailty that it cannot bear to gaze on such great glory." But men are healed of this frailty by Christ when He brings them into glory. And this is signified by what He says to them: "Arise, and fear not."
Source: Summa Theologica (New Advent)