1 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 has testified, saying, No man has 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 says 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 You shall rend the heavens, trembling will take hold of the mountains at sight of You, and they will flow down. And why do you 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?
Source: Catechetical Lectures (New Advent)