Cyril of Alexandria

How then should men suppose Him who is really the Son to be made or created, when God the Father thundered from above, This is my beloved Son! as if He said, Not one of My sons, but He who is truly and by nature My Son, according to whose example the others are adopted, He ordered them then to obey Him, when He added, Hear ye him. And to obey Him more than Moses and Elias, for Christ is the end of the Law and the Prophets. Hence the Evangelist adds significantly, And when the voice was past, Jesus was found alone.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Lest in truth any one should suppose that these words, This is my beloved Son, were uttered about Moses or Elias.

Catena Aurea by Aquinas ↗

Ambrose of Milan

Now observe, that the cloud was not black from the darkness of condensed air, and such as to overcast the sky with a horrible gloom, but a shining cloud, from which we were not moistened with rain, but as the voice of Almighty God came forth the dew of faith was shed upon the hearts of men. For it follows, And there came a voice out of the cloud, saying, This is my beloved Son: hear ye him. Elias was not His Son. Moses was not. But this is the Son whom you see alone.

Catena Aurea by Aquinas ↗

Bede

And they were afraid as they entered into the cloud, and a voice came from the cloud, saying: This is my beloved Son, hear him. Human fragility cannot bear the sight of greater glory, and trembling with the whole mind and body, it falls to the ground. The more one seeks greater things, the more he collapses to lower things, if he does not know his own measure. …

On the Gospel of Luke ↗

Bonaventure

Second, as regards the revelation of credible truth, it is added: And as they entered into the cloud, namely, the cloud arousing them to reverence, according to what is said in Exodus 24: "Moses, entering into the midst of the cloud, ascended the mountain," where he heard the divine revelations. …

Commentary on Luke, Chapter 9 ↗