John Chrysostom

Having taken therefore the leaders, "He bringeth them up into a high mountain apart, and was transfigured before them: and His face did shine as the sun, and His raiment was white as the light. And there appeared unto them Moses and Elias talking with Him." But wherefore doth He also bring forward Moses and Elias? One might mention many reasons. …

Homily on the Gospel of Matthew 56 ↗

Augustine of Hippo

Indeed, Jesus himself shone as the sun, indicating that he is the light which illuminates every one who comes into this world. And this is the sun to the eyes of the flesh, that is the sun to the eyes of the heart. His garments are a type of his church. For garments, unless held up by the one having donned them, fall. Paul was like the lowest hem of these garments. …

Sermon 78.2 ↗

Jerome

And he was transfigured before them. Just as he will be at the time of judgment, so he appeared to the apostles. But when it says, 'He was transfigured before them,' no one can think that he lost or abandoned his original form and appearance, or that he took on a spiritual or aerial body. Rather, the evangelist demonstrates how he was transformed, saying. And his face shone like the sun, and his garments became white as snow. …

Commentary on Matthew ↗

Thomas Aquinas

Then follows the transfiguration: and he was transfigured before them. And first the transfiguration is presented; secondly, the testimony, at while he was yet speaking, etc. Concerning the first, the transfiguration is presented; secondly, the manner; thirdly, Peter's admiration. He says therefore and he was transfigured, i.e., he changed his figure, before them. To be transfigured is the same as to be changed from one's proper figure, as is found in 2 Corinthians 11, that Satan transforms himself into an angel of light. …

Commentary on Matthew ↗