We don’t really know what happened when the angels got divided into light and dark. It’s not something made completely clear in scripture, although of course everyone speculates. Revelation 12 states clearly that war broke out in Heaven and the dragon (ie, Satan) swept a third of the stars down from the sky. We assume God posed a test to the angels, and whatever it was, it was so tough that a third of them couldn’t do it. Or wouldn’t.
There’s a passage in Ezekiel about how Lucifer tried to raise his throne higher than God’s, but I can’t imagine it was a literal rebellion along the lines of “You’ve been king long enough; now it’s my turn.” They knew God was omnipotent. I think it was more along the lines of, “I don’t want to do what you want me to do. I want to stand alone.”
If you look at the context of Revelation 12, the rebellion of the angels is placed immediately beside the depiction of the woman clothed with the sun, the holy tabernacle. And she gives birth to a child that is clearly supposed to be Jesus. This led the church fathers to speculate that it was the incarnation of Jesus as a man which caused the angels to rebel: they didn’t want to worship God incarnate in a lower form, man being “a little less than the angels” according to the psalm.
But what it if wasn’t Jesus specifically? What if it was just the idea that God would breathe His life into matter, into an animal thing?
It doesn’t seem offensive to us because we are animal things. We’re used to us. But if you’re an angel and you’re used to thinking of matter as something you play with, something you see as a reflection of God’s glory but not really glorious in itself, that’s got to be a shock. It’s absurd. It would be the same thing as you coming home to find your mother kissing a pig, and then she sets a place for it at the table and says the two of you will be sharing a bedroom. Why are you doing that? What are you trying to prove? That’s just wrong!
What if they felt God’s holiness was so great, so pure, so tremendous, that cramming His life into a temporary biological construct (smelly, awkward, limited) was an affront to the order of things? What if they knew they’d have to love the Spirit of God in such a monstrous hybrid, unless they stopped loving God completely?
It’s the equivalent of “I can’t bear to see you this way, so I won’t see you at all.”
And a third of them couldn’t.
We don’t know; we’ll never know in this lifetime, and it’s possible we’re better off not knowing.