9 And the Word spoke to Moses, appearing before him, “just as any one might speak to his friend.” But Moses desired to see Him openly who was speaking with him, and was thus addressed: “Stand in the deep place of the rock, and with My hand I will cover you. But when My splendour shall pass by, then you shall see My back parts, but My face you shall not see: for no man sees My face, and shall live.” Two facts are thus signified: that it is impossible for man to see God; and that, through the wisdom of God, man shall see Him in the last times, in the depth of a rock, that is, in His coming as a man. And for this reason did He [the Lord] confer with him face to face on the top of a mountain, Elias being also present, as the Gospel relates, He thus making good in the end the ancient promise.
Source: Adversus haereses (New Advent)