Arizaphale had tried to explain it to him once. The whole point was that when a human was good or bad it was because they wanted to be. Whereas people like Crowley and, of course, himself, were set in their ways right from the start. People couldn’t become truly holy unless they also had the opportunity to be definitely wicked.
Crowley had thought about this for some time and had said, Hang on, that only works if you start everyone off equal, OK? You can’t start someone off in a muddy shack in the middle of a war zone and expect them to do as well as someone born in a castle.
Ah, Arizaphale had said, that’s the good bit. The lower you start, the more opportunities you have.
Crowley had said, That’s lunatic.
No, said Arizaphale, it’s effable.
Terry Pratchett & Neil Gaiman