Excerpt from St. Gregory Palamas’ Homily on the Transfiguration of the Lord (you can read the homily in its entirety here).
“What does it mean to say: He was transfigured?” asks the Golden-Mouthed Theologian (Chrysostomos). He answers this by saying: “It revealed something of His Divinity to them, as much and insofar as they were able to apprehend it, and it showed the indwelling of God within Him.” The Evangelist Luke says: “And as He prayed, His countenance was altered” (Lk 9:29); and from the Evangelist Matthew we read: “And His face shone as the sun” (Mt 17:2). But the Evangelist said this, not in the context that this Light be thought of as subsistent for the senses (let us put aside the blindness of mind of those who can conceive of nothing higher than what is known through the senses). Rather, it is 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 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? But he (Moses) did not effect this, but rather he underwent a transfiguration. However, our Lord Jesus Christ possessed that Light Himself. In this regard, actually, He 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. On Mount Tabor it was manifest also in His Flesh, by reason of the Hypostatic Union (i.e., the union of the two perfect natures, divine and human, within the divine Person [Hypostasis] of Christ, the Second Person of the Most Holy Trinity). The Fourth Ecumenical Council at Chalcedon defined this Hypostatic union of Christ’s two natures, divine and human, as “without mingling, without change, without division, without separation.”
We believe that at the Transfiguration He manifested not some other sort of light, but only that which was concealed beneath His fleshly exterior. 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. Rather, it was to show His disciples that which He already was, opening their eyes and bringing them from blindness to sight. For do you not see that eyes that can perceive natural things would be blind to this Light?