that optimist crowley quote from the book got me thinking, because he kind of Has to be. in order for a demon's job to be necessary, you have to believe that human nature is inherently Good in order to believe it's corruptible. likewise with aziraphale and angels, you have to believe that human nature is inherently Wicked in order to believe it needs saving.

in fact, i think what makes them kind of give up on their jobs is that they're given ample opportunity for crowley to see that humans come up with wickedness all on their own and for aziraphale to see that humans do wonderfully kind things all on their own, which goes against their own inherent beliefs but fits right in with each other's.