ST. CYRIL OF JERUSALEM, CATECHETICAL LECTURES, 9,1-3
"To look upon God with eyes of flesh is impossible: for the incorporeal cannot be subject to bodily sight: and the Only begotten Son of God Himself hath testified, saying, No man hath seen God at any time. For if according to that which is written in Ezekiel any one should understand that Ezekiel saw Him, yet what saith the Scripture? He saw the likeness of the glory of the Lord; not the Lord Himself, but the likeness of His glory, not the glory itself, as it really is. And when he saw merely the likeness of the glory, and not the glory itself, he fell to the earth from fear.
Now if the sight of the likeness of the glory brought fear and distress upon the prophets, any one who should attempt to behold God Himself would to a certainty lose his life, according to the saying, No man shall see My face and live. For this cause God of His great loving-kindness spread out the heaven as a veil of His proper Godhead, that we should not perish. The word is not mine, but the Prophet's. If Thou shalt rend the heavens, trembling will take hold of the mountains at sight of Thee, and they will flaw down. And why dost thou wonder that Ezekiel fell down on seeing the likeness of the glory? when Daniel at the sight of Gabriel, though but a servant of God, straightway shuddered and fell on his face, and, prophet as he was, dared not answer him, until the Angel transformed himself into the likeness of a son of man. Now if the appearing of Gabriel wrought trembling in the Prophets, had God Himself been seen as He is, would not all have perished?
The Divine Nature then it is impossible to see with eyes of flesh: but from the works, which are Divine, it is possible to attain to some conception of His power, according to Solomon, who says, For by the greatness and beauty of the creatures proportionably the Maker of them is seen. He said not that from the creatures the Maker is seen, but added proportionably. For God appears the greater to every man in proportion as he has grasped a larger survey of the creatures: and when his heart is uplifted by that larger survey, he gains withal a greater conception of God.
Wouldest thou learn that to comprehend the nature of God is impossible? The Three Children in the furnace of fire, as they hymn the praises of God, say Blessed art thou that beholdest the depths, and sittest upon the Cherubim. Tell me what is the nature of the Cherubim, and then look upon Him who sitteth upon them. And yet Ezekiel the Prophet even made a description of them, as far as was possible, saying that every one has four faces, one of a man, another of a lion, another of an eagle, and another of a calf; and that each one had six wings, and they had eyes on all sides; and that under each one was a wheel of four sides. Nevertheless though the Prophet makes the explanation, we cannot yet understand it even as we read. But if we cannot understand the throne, which he has described, how shall we be able to comprehend Him who sitteth thereon, the Invisible and Ineffable God? To scrutinise then the nature of God is impossible: but it is in our power to send up praises of His glory for His works that are seen."