Anyone could have seen the truth of this from the Word—for instance, from the time when the Lord revealed himself to Peter, James, and John, since his face then shone like the sun and his clothes became like the light (Matthew 17:2). The only reason he appeared this way to them was that their inner eyes were open.
The prophets provide evidence for the same thing. In Isaiah, for example, where it deals with the Lord’s kingdom in the heavens:
The light of the moon will be like the light of the sun, and the light of the sun will be seven times as strong, like the light of seven days. (Isaiah 30:26)
And in a passage from John also dealing with the Lord’s kingdom, which is called the New Jerusalem:
The city has no need for the sun or the moon to shine in it, since God’s glory lights it and its lamp is the Lamb. (Revelation 21:23)
In another place as well:
Night will not exist there, and they will have no need for a lamp or sunlight, because the Lord God gives them light. (Revelation 22:5)
Additionally, when the Lord appeared to Moses, Aaron, Nadab, Abihu, and the seventy elders,
They saw the God of Israel, under whose feet was something like a work of sapphire stone, and it looked like the substance of the sky for cleanness. (Exodus 24:10)
Since the Lord’s heavenly, spiritual quality appears to angels’ outward sight as a sun or moon, the sun in the Word symbolizes something heavenly; the moon, something spiritual.
from Secrets of Heaven, Volume 2, Section 1530