10 Make an effort, and do not grow weary of my prolonged discourse. For as when He manifests Himself, He is not manifested as He really is, nor is His bare essence manifested (for no man has seen God in His real nature; for when He is but partially revealed the Cherubim tremble— the mountains smoke, the sea is dried up, the heaven is shaken, and if the revelation were not partial who could endure it?) as then, I say, He does not manifest Himself as He really is, but only as the beholder is able to see Him, therefore does He appear sometimes in the form of old age, sometimes of youth, sometimes in fire, sometimes in air, sometimes in water, sometimes in armour, not altering his essential nature, but fashioning His appearance to suit the various condition of those who are affected by it.
In like manner also when any one wishes to say anything concerning Him he employs human illustrations. For instance I say: “He went up into the mountain and He was transfigured before them, and His countenance shone as the sun, and His raiment became white as snow.” He disclosed, it is said, a little of the Godhead, He manifested to them the God dwelling among them “and He was transfigured before them.” Attend carefully to the statement. The writer says and He was transfigured before them, and His raiment shone as the light, and His countenance was as the sun.
When I said “such is His greatness and power” and added “be merciful to me O Lord,” (for I do not rest satisfied with the expression but am perplexed, having no other framed for the purpose) I wish you to understand, that I learned this lesson from Holy Scripture. The evangelist then wished to describe His splendour and he says “He shone.” How did He shine? Tell me. Exceedingly. And how do you express this? He shone “as the sun.” As the sun do you say? Yea. Wherefore? Because I know not any other luminary more brilliant.
And He was white do you say as snow? Wherefore as snow? Because I know not any other substance which is whiter. For that He did not really shine thus is proved by what follows: the disciples fell to the ground. If he had shone as the sun the disciples would not have fallen; for they saw the sun every day, and did not fall: but inasmuch as he shone more brilliantly than the sun or snow, they, being unable to bear the splendour, fell to the earth.
Source: Second Homily on Eutropius (After His Captivity) (New Advent)