Moses entered into the darkness and there he saw God.
What does this signify? This present account seems in a way to contradict that of the first theophany.
Then God appeared in light, but now he appears in darkness.
Yet we must not imagine this to be at variance with our normal experience of spiritual contemplation.
By this statement the text teaches us that religious knowledge is first experienced as light.
All that is seen to be opposed to religion is darkness, and darkness vanishes when we receive the light.
But the more the mind advances and by ever increasing and more perfect application attains an intellectual comprehension of realities and approaches contemplation, the more clearly it sees that the divine nature is invisible.
Having left behind all appearances, not only those perceived by the senses but also those the intellect seems to see, it plunges ever deeper within itself, until by spiritual effort it penetrates to the invisible and the unknowable, and there it sees God.
This is the true knowledge of what is sought; this is the seeing that consists in not seeing, because that which is sought transcends all knowledge, being separated on all sides by incomprehensibility as by a kind of darkness.
This is why John the contemplative, who had penetrated this luminous darkness, said that no one had ever seen God, declaring by this negation that the divine essence is beyond the reach not only of men but of every rational nature as well.
And so, when Moses had advanced 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.
The text says: Moses entered into the darkness where God was.
What God? He who has made the darkness his covering, as David declared, who had himself been initiated into the divine mysteries in that same sanctuary.
When Moses arrived there, he was taught by word what he had formerly learned from darkness, so that, I think, the doctrine on this matter may be made more firm for us by the witness of the divine voice.
The divine word at the beginning forbade that the Divine be likened to any of the things known by men, since every concept which comes from some comprehensible image constitutes an idol of God and does not proclaim God.
Gregory of Nyssa (c 335 – after 394): The Life of Moses, 2.162-66 (SC 1, 80-82); from the Monastic Office of Vigils, Friday of the Second Week in Lent, Year 2.