Moses receives three successive visions of God: first he sees God in a vision of light at the burning bush (Ex. 3:2); next God is revealed to him through mingled light and darkness, in the "pillar of cloud and fire" which accompanies the people of Israel through the desert (Ex. 13:21); and then finally he meets God in a "non-vision", when he speaks with him in the "thick darkness" at the summit of Mount Sinai (Ex. 20:21).First, He shows me that I can know Him. Then, He shows me that I cannot define Him. First, He shows me that He is. Then, He shows me that He is more. First, He shows me that He is Truth. Then, He shows me that He is Mystery.
Kallistos Ware, in the chapter "God as Mystery" from "The Orthodox Way", St Vladimir's Seminary Press: 1979. 13