29 But what shall we say about the Lord of the Angel, Who said to Moses— No man shall see Me and live? Is it on account of the fury of His anger, that whoso shall see Him shall die? Or on account of the splendour of His Being? For that Being was not made and was not created: so that eyes which have been made and created cannot look upon it. For if it is on account of His fury that whoso shall look upon Him shall not live, lo! He would have granted to Moses to see Him because of His great love to him.
Accordingly, the Self-Existent by His vision slays them that look upon Him; but He slays, not because of harsh fury but because of His potent splendour. Because of this He in His great love granted to Moses to see His glory; yet in the same great love He restrained him from seeing His glory. But it was not that the glory of His majesty would have been at all diminished, but that weak eyes could not suffice to bear the overpowering billows of His glory. Therefore God, Who in His love desired that the vision of Moses should be directed upon the goodly brightness of His glory, in His love did not desire that the vision of Moses should be blinded amidst the potent rays of His glory.
Therefore Moses saw and saw not. He saw, that he might be exalted; he saw not, that he might not be injured. For by that which he saw, his lowliness was exalted; and by that which he saw not, his weakness was not blinded. As also our eyes look upon the sun and look not upon it; and by what they see are assisted; and by what they see not, are uninjured. Thus the eye sees, that it may be benefited; but it ventures not [to look], that it may not be injured. So then through love God hindered Moses from seeing that glory that was too hard for his eyes: As also Moses through his love prevented the children of his people from seeing the brightness that was too strong for their eyes.
For he learned from Him Who covered him, and spread His hand, and hid from him the splendour of the glory, that it might not injure him; so that he also should spread the veil and conceal from the feeble ones the overpowering splendour, that it might not hurt them. Now when Moses saw that the sons of perishable flesh could not gaze upon the borrowed glory that was on his face, his heart failed within him; for that he had sought to dare to gaze upon the glory of the Eternal Being; in whose floods, lo! Those above and those below are plunged and spring forth; the depths whereof none can fathom; the shores whereof none can reach; whereof no end or limit can be found.
Source: On Our Lord (New Advent)