St. Symeon the New Theologian (949-1022) – One of the great Orthodox mystics and hesychasts explains his experience and encounter of God as light.
“This, invariably, is just what occurs concerning the invisible God. Whenever someone sees Him revealed, he sees light. While on the one hand he is amazed at what he has seen, on the other he does not know immediately who it is who has appeared, yet he dares not ask Him. And how could he? He is unable even to lift up his eyes and look on that grandeur. With fear and trembling he looks instead, as it were, at his own feet, knowing fully only that it is Someone Who has appeared before his face.
And if there happens to be some other man who has told him beforehand about such things, as having known God from before, he goes to this man [St Symeon’s elder] and says: “I have seen.” And the other says: “What did you see, child?” “Light, O my father, so sweet, sweet! So much so, father, that my reason has not the strength to tell you.”
And, while he is saying this, his heart leaps and pounds, and catches on fire with longing for what he has seen. Then, with many warm tears, he begins to say again: “That light, father, appeared to me. The walls of my cell immediately vanished and the world disappeared, fleeing I think from before His face, and I remained alone in the presence alone of the light. And I do not know, father, if this my body was there, too. I do not know if I was outside of it. For a while I did not know that I carry and am clothed with a body. And such great joy was in me and is with me now, great love and longing both, that I was moved to streams of tears like rivers, just like now as you see.”
The other then answers and says: “It is He, child.” And, at this word, he sees Him again and, little by little, comes to be completely purified and, purified, grows bold and asks that One Himself, and says: “My God, is it You?”
And He answers and says: ‘Yes, I am He, God, Who for your sake became man; and behold, I have made you, as you see, and shall make you, god’.”
~ St Symeon the New Theologian, from On the Mystical Life (Vol. 2), pp. 53-54