[…] Neither is it called Day of Judgment because it is on this day that judgment is going to take place, since the day when this occurs is not other than the Lord who will come on it, but it is called this because He Himself, the God and Master of all, will at that time shine with the glory of His own divinity.
[…] And He alone will be at once “Day” and God. He Who is now invisible to all and dwells in light will then be revealed to all as He is, and will fill all things with His light, and will be without evening, without end, a day of everlasting joy, but absolutely unapproachable and unseen for those who, like me, are lazy and sinners.
Because this did not happen while they yet lived, because they lacked zeal to see the light of His glory and, through purification, to have Him completely indwelling in themselves, He will also naturally be unapproachable for them in the future.
[…] The revelation of His divinity becomes in fact a judgment for those to whom it is revealed. No flesh could have endured the glory of His divinity as manifested naked of its joining and inexpressible union with the God-man. All creation would instead have been utterly destroyed both in body and soul, since at that time all were possessed by unbelief.
For the divinity, which is to say the grace of the all-Holy Spirit, has never appeared to anyone who is without faith; and, if it were to appear by some paradox among men, it would show itself as fearful and dreadful, as not illumining but burning, not as giving life but as punishing dreadfully.
And this is clear from the things which the blessed Paul, the vessel of election suffered. In the encounter with the radiance of the unapproachable light which flashed around him like lightning, his vision was wounded, and rather than being illumined he was darkened. He could not see, and lost even his natural faculty of sight.
These things happened to him who would later become the great teacher of Christ’s Church! That man who was so great, the same man who later said: ‘The God Who said “Let light shine out of darkness” has shone in our hearts,’ and a little later: ‘We have this treasure’ — i.e., of illumination — ‘in our hearts’ could not at that time see even the least glimmer of the light.
Symeon the New Theologian (949–1022 AD): Tenth Ethical Discourse @ Eclectic Orthodoxy.