“For we did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty. For when He received honor and glory from God the Father and the voice was borne to Him by the Majestic Glory, “This is my beloved Son, with whom I am well pleased,” we heard this voice borne from heaven, for we were with Him on the holy mountain. And we have the prophetic word made more sure. You will do well to pay attention to this as to a lamp shining in a dark place, until the day dawns and the morning star rises in your hearts (2 Pet. 1:16-19 RSVCE).”
Leo the Great: “For when the Father said, ‘This is My beloved Son, in Whom, I am well pleased; listen to Him,’ was it not clearly meant, ‘This is My Son’, Who is eternally from Me and with Me? ‘This is My Son’, not adopted, but true-born, not created from another source, but begotten of Me, nor made like Me from another nature, but born equal to Me of My nature (John 1:14). ‘This is My Son’, through Whom all things were made, and without Whom was nothing made (John 1:3). All things that I do He does in like manner, and whatever I perform, He performs with Me inseparably (John 5:19) and without difference: for the Son is in the Father and the Father in the Son (John 14:11) and Our Unity is never divided. (John 10:30).
“Listen to Him, therefore, because He is the Truth and the Life (John 14:6), He is My Power and Wisdom (1 Cor. 1:24). ‘Listen to Him’, Who redeems the world by His blood (Eph. 1:7), Who binds the devil and carries off his captives (Matt. 12:28-29), Who destroys the bond of sin [and sets you free] (John 8:34-36). ‘Listen to Him’, Who opens the way to heaven, and by the… cross prepares for you the steps of ascent to the Kingdom.
“These things, dearly beloved, were said not only for those who heard them with their own ears, but in these three Apostles the whole Church has learned all that their eyes saw and their ears heard. Let all men’s faith be established according to the preaching of the most holy Gospel, and let no one be ashamed of Christ’s cross through which the world was redeemed.” (Leo the Great, 440-461 A.D., excerpts from Homily 51, On the Transfiguration – portions of a previous post)