The Evangelist said this…to show that Christ-God, for those living and contemplating by the Spirit, is the same as the sun is for those living in the flesh and contemplating by the senses.
Therefore, some other Light for the knowing the Divinity is not necessary for those who are enriched by Divine gifts.
That same Inscrutable Light shone and was mysteriously manifest to the Apostles and the foremost of the Prophets at that moment, when the Lord was praying.
This shows that what brought forth this blessed sight was prayer, and that the radiance occurred and was manifest by uniting the mind with God.
And it shows that it is granted to all who, with constant exercise in efforts of virtue and prayer, strive with their mind towards God.
True beauty, essentially, can be contemplated only with a purified mind.
To gaze upon its luminance assumes a sort of participation in it, as though some bright ray etches itself upon the face.
Even the face of Moses was illumined by his association with God. Do you not know that Moses was transfigured when he went up the mountain, and there beheld the Glory of God?
Moses did not effect this, but rather he underwent a transfiguration. However, our Lord Jesus Christ possessed that Light Himself….
Christ did not need prayer for His flesh to radiate with the Divine Light; it was but to show from whence that Light descends upon the saints of God, and how to contemplate it.
For it is written that even the saints “will shine forth like the sun” (Mt 13:43), which is to say, entirely permeated by Divine Light as they gaze upon Christ, divinely and inexpressibly shining forth His Radiance, issuing from His Divine Nature.
[...] This Light was the Light of the Divine Nature, and as such, it was Uncreated and Divine.
So also, in the teachings of the Fathers, Jesus Christ was transfigured on the Mount, not taking upon Himself something new nor being changed into something new, nor something which formerly He did not possess.
[...] This Light is not a light of the senses, and those contemplating it do not simply see with sensual eyes, but rather they are changed by the power of the Divine Spirit.
They were transformed, and only in this way did they see the transformation taking place amidst the very assumption of our perishability, with deification through union with the Word of God in place of this.
Gregory Palamas (1296-1359): extracted from Homilly on the Transfiguration (from the translation at Pravoslavie.ru).