The Father is light, the Son is light, the Holy Spirit is light.
Watch what you say, brother, watch lest you go astray!
For the Three are one light, one, not separated,
but united in three persons without confusion.
For God is wholly undivided by nature
and in essence He is truly beyond all essence.
He is not split in power, nor in form, nor in glory,
nor in appearance, for He is contemplated entirely as simple light.
In these the persons are one, the three hypostases are one.
For the Three are in the one, or rather the Three are one,
the Three are one power, the Three are one glory,
the Three are one nature, one essence, and one divinity.
And these are the one light that illuminates the world,
not the world, perish the thought, not this visible world
—for this visible world has not known Him, nor is it
able to know, nor can the friends of the world,
for the one who loves the world is an enemy of God,
but we call “the world” that which God has made human
according to his image and likeness,
because one is adorned with virtues, one rules terrestrial beings;
just as God has authority over the universe,
so also one reigns over the passions according to this image,
and subjugates demons, the craftsmen of evils,
and tramples underfoot the dragon, the primeval, the huge
dragon like a common sparrow. And how? Listen child!
This fallen prince immediately found himself in darkness
because he was deprived of the light; he is now in darkness
with all those who fell with him from heaven;
he reigns in it—certainly in the darkness I say—
over demons and humans who are held in the darkness.
Every soul who does not see the light of life shining
both in the day and night is punished by the prince of darkness:
wounded, subdued, dragged, and enchained,
and stabbed daily by the darts of pleasure.
Even if the soul seems to resist, even if she seems not to fall,
but still she always has an irreconcilable war with him
in much sweat, toil, trouble, and hardship.
But every soul who contemplates the divine light,
from whence the evil prince has fallen, despises the evil one,
and once enlightened by the unapproachable light itself,
then the soul tramples underfoot the prince of darkness like a leaf
fallen on the ground from a high tree.
For she is in darkness where he has power and authority,
but in the light he becomes an utterly dead corpse.
Symeon the New Theologian (949–1022 AD): from Hymn 33.