GREGORY OF NYSSA (c. 355 AD - c. 394)
This truly is the vision of God: never to be satisfied in the desire to see God. But we must always, by looking at what we can see, rekindle our desire to see more. Thus, no limit would interrupt growth in the ascent to God, since no limit to the Good can be found, nor is the increasing of desire for the Good brought to an end because it is satisfied....
When the Lord who spoke to Moses came to fulfil his own law, he gave a clear explanation to his disciples, laying bare the meaning of what had previously been said figuratively when he said, “If anyone want to be a follower of mine…” (Luke 9:23), and not, ‘If anyone wants to go before me.’ And to the one asking about eternal life he proposes the same thing, for he says “Come follow me” (Luke 18:22). Now, the one who follows sees the back.
So Moses, who eagerly seeks to behold God, is now shown how: to follow God wherever he might lead is to behold God. God's passing by signifies his guiding the one who follows, for someone who does not know the way cannot complete his journey safely in any other way than by following behind his guide. He who leads, then, by his guidance shows the way to the one following. The one who follows will not turn aside from the right way if he or she always keeps the back of the leader in view.
For those who move to one side or bring themselves to face their guide assume another direction for themselves than the one the guide shows them. Therefore he says, “My face is not to be seen” (Exod. 33:23), that is, ‘Do not face your guide.’ If they do so, their course will certainly be in the opposite direction, for good does not look good in the face, but follows it.
What is perceived to be its opposite is face to face with the good, for what looks virtue in the face is evil. But virtue is not perceived in contrast to virtue. Therefore Moses does not look God in the face, but looks at his back; for whoever looks at him face to face shall not live, as the divine voice testifies, “You cannot see my face and live” (Exodus 33:20).