What does it mean that Moses entered the darkness and then saw God in it? (Exodus 20:21). What is not recounted seems somehow to be contradictory to the first theophany, for then the Divine was beheld in light but now he is seen in darkness. Let us not think that this is at variance with the sequence of things we have contemplated spiritually. Scripture teaches by this that religious knowledge comes at first to those who receive it as light. Therefore what is perceived to be contrary to religion is darkness, and the escape from darkness comes about when one participates in light. But as the mind progresses and, through an ever greater and more perfect diligence, comes to apprehend reality, as it approaches more nearly to contemplation, it sees more clearly what of the divine nature is uncontemplated.
For leaving behind everything that is observed, not only what sense comprehends but also what the intelligence thinks it sees, it keeps on penetrating deeper until by the intelligence’s yearning for understanding it gains access to the invisible and the incomprehensible, 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. Wherefore John the sublime, who penetrated into the luminous darkness, says, “No one has ever seen God” (John 1:18), thus asserting that knowledge of the divine essence is unattainable not only by human beings but also by every intelligent creature.
When, therefore, Moses grew in knowledge, he declared that he had seen God in darkness, that is, that he had then come to know that what is divine is beyond all knowledge and comprehension, for the text says, “Moses approached the dark cloud where God was” (Exodus 20:21). What God? He who “made darkness his hiding place” as David says (Ps 18:11), who also was initiated into the mysteries in the same inner 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.
From St. Gregory of Nyssa, Life of Moses