When it pleases the Lord, good spirits appear to others (and also to themselves) as shining stars whose radiance matches the quality of the spirits’ charity and faith. Evil spirits, though, resemble little balls of coal fire.
Sometimes, among evil spirits, a life devoted to the fulfillment of one’s sensual desires resembles a coal fire. This semblance of fire is what the vitality of the Lord’s love and mercy turns into when it flows into them. The vital energy of their hallucinations, on the other hand, looks like the light that comes from such a fire—a dim light with no range whatever. When the living energy of mutual love approaches, the pseudo-fire is snuffed out and turns cold, while the weak light from it goes dark.
Evil spirits pass their lives in the dark. Astonishingly, some of them even love darkness and abominate light.
People know perfectly well in heaven—though not as well in the world of spirits—where such great light comes from: the Lord. Remarkably, in the third heaven the Lord appears to heavenly angels as a sun and to spiritual angels as a moon. There is no other source for their light. But the amount of light they enjoy depends on the amount of heavenliness and spirituality they have; and the type of light they enjoy depends on the type of heavenliness and spirituality. Consequently, the genuinely heavenly and spiritual quality of the Lord presents itself to the angels’ outward sight in the form of light.
from Secrets of Heaven, Sections 1527-1529