“Moses drew near to the thick darkness where God was” (Exodus 20:21).
Moses entered into the darkness and there he saw God....
John the contemplative, who had penetrated this luminous darkness, says, “No one has ever seen God,” declaring by this negation that the divine essence is beyond the reach not only of humans but of every rational nature as well. And so, when Moses grew in knowledge, he declared that he saw God in the darkness, or in other words that he recognized that the Divinity is essentially that which transcends all knowledge and which no mind can apprehend.
For the text says: “Moses entered into the darkness where God was.” What God? He who “made the darkness his hiding place,” as David declared, who was also initiated into the divine mysteries in that same inner sanctuary.
Gregory of Nyssa, The Life of Moses (4th century)