John of Damascus — as excerpted in the Catena Aurea on Luke 9:28-31

“(ut sup. 13.) Now the devil, seeing His face shining in prayer, recollected Moses, whose face was glorified. But Moses indeed was arrayed with a glory, which came from without; our Lord, with that which proceeded from the inherent brightness of Divine glory. (Exod. 34:29.) For since in the hypostatical union there is one and the same glory of the Word and the flesh, He is transfigured not as receiving what He was not, but manifesting to His disciples, what He was. Hence, according to Matthew, it is said, that He was transfigured before them, and that His face shone as the sun; (Mat. 17:2.) for what the sun is in things of sense, God is in spiritual things. And as the sun, which is the fountain of light, cannot be easily seen, but its light is perceived from that which reaches the earth; so the countenance of Christ shines more intensely, like the sun, but His raiment is white as snow; as it follows, And his raiment was white and glistering; that is, lighted up by its participation of the divine light. And a little afterwards, But while these things were so, that it might be shewn there was but one Lord of the new and old covenant, and the mouths of heretics might be shut, and men might believe on the resurrection, and He also, who was transfigured, be believed to be the Lord of the living and the dead, Moses and Elias, as servants, stand by their Lord in His glory; hence it follows, And behold there talked with him two men. For it became men, seeing the glory and confidence of their fellow servants, to admire indeed the merciful condescension of the Lord, but to emulate those who had laboured before them, and looking to the pleasantness of future blessings, to be the more strengthened for conflicts. For he who has known the reward of his labours, will the more easily endure them.”