7 Moreover, that you may be sure that this is He who was seen of Moses, hear Paul's testimony, when he says, For they all drank of a spiritual rock that followed them; and the rock was Christ. And again: By faith Moses forsook Egypt, and shortly after he says, accounting the reproach of Christ greater riches than the treasures in Egypt. This Moses says to Him, Show me Yourself. You see that the Prophets also in those times saw the Christ, that is, as far as each was able. Show me Yourself, that I may see You with understanding. But He says, There shall no man see My face, and live. For this reason then, because no man could see the face of the Godhead and live, He took on Him the face of human nature, that we might see this and live.
And yet when He wished to show even that with a little majesty, when His face did shine as the sun, the disciples fell down affrighted. If then His bodily countenance, shining not in the full power of Him that wrought, but according to the capacity of the Disciples, affrighted them, so that even thus they could not bear it, how could any man gaze upon the majesty of the Godhead? 'A great thing,' says the Lord, you desire, O Moses: and I approve your insatiable desire, and I will do this thing for you, but according as you are able. Behold, I will put you in the cleft of the rock: for as being little, you shall lodge in a little space.'
Source: Catechetical Lectures (New Advent)