24 And again, because the life of believers was involved in the confession of this faith—for there is no other way to eternal life than the assurance that Jesus Christ, God the Only-begotten, is the Son of God— the Apostles heard once more the voice from heaven repeating the same message, in order to strengthen this life-giving belief, in negation of which is death. When the Lord, apparelled in splendour, was standing upon the Mountain, with Moses and Elias at His side, and the three Pillars of the churches who had been chosen as witnesses to the truth of the vision and the voice, the Father spoke thus from heaven:— This is My beloved Son in Whom I am well pleased; hear Him. The glory which they saw was not sufficient attestation of His majesty; the voice proclaims, This is My Son.
The Apostles cannot face the glory of God; mortal eyes grow dim in its presence. The trust of Peter and James and John fails them, and they are prostrate in fear. But this solemn declaration, spoken from the Father's knowledge, comes to their relief; He is revealed as His Father's own true Son. And over and above the witness of This and My to His true Sonship, the words are uttered, Hear Him. It is the witness of the Father from heaven, in confirmation of the witness borne by the Son on earth; for we are bidden to hear Him.
Though this recognition by the Father of the Son removes all doubt, yet we are bidden also to accept the Son's self-revelation. When the Father's voice commands us to show our obedience by hearing Him, we are ordered to repose an absolute confidence in the words of the Son. Since, therefore, the Father has manifested His will in this message to us to hear the Son, let us hear what it is that the Son has told us concerning Himself.
Source: On the Trinity (New Advent)