It's rather straightforward:
GOAL: Get the world population to accept literal demons.
STRATEGY: Flood the masses' consciousness with the idea of ALIENS (movies (Spielberg)), then do the big reveal, but simply call the demons aliens.
If not demons, would also work with human-animal hybrids that they've been creating (because they end up looking alien--at least how they've programmed us to imagine aliens).
If they're willing to experiment on animals
and on humans
you think they're NOT going to experiment by breeding humans with animals?
animal-human hybrids
There's a reason they're conditioning women to be in love with vampires (bat-men), werewolves, fishmen, and generic beasts.
(of course, they used Tom Hanks to get boys to have the hots for half woman-half fish mermaids)