An idea I had a while back.
Succubi. Demons. Fallen angels.
War in Paradise. Angels who defied the will of the LORD were cast out, to spend eternity following their new leader, Lucifer, He who fell furthest, He who fell first.
For a while at least, all was well with the underworld. Eventually, though, some of the fallen began to dissent. They knew they were damned, yes - they could never again know the embrace of the Divine, a light and a softness and complete love, but they should not have to follow the will of something utterly evil because they had wavered once in their devotion to Good.
It took centuries for these thoughts to gather enough strength to spur them into action, but eventually the Succubi and Incubi, the demons who spent the most time contemplating the actions of Man, were united in their opposition to Satan. They could never be forgiven - the LORD had spoken, and his word is infallible, but they could show Him that they were not a lost cause, and perhaps be granted His love once more. Even if they could not, it was the right thing to do - Angels can fall from grace; Demons can rise from corruption.
It was the ultimate sacrifice. Giving up their place in Hell, with nowhere to wander but the earth. They could be destroyed here, a fate worse than any damnation, but it was here they could do their work. Slipping into positions of power, guiding the human race in the works of the LORD, shielding them from sin. Those who understood so perfectly how to make humans turn to sin, turn away from the LORD, they were best equipped to undo their own work and make humanity flourish under the banner of the Lamb.
TL;DR alternative industrial revolution with divine magic, a ruling caste of demons, and the humans who make this ultimate sacrifice and become immortal, unable to enter heaven, to help ensure that most ascend to Paradise.