"Uncle Humblestaff?" the young hero responded. "Of course I know the man. Wouldn't have had a chance at the academy without a word from him. He's the big guy over there, surrounded by all those adventurers."
The only remnant of the Shattering, Lydecker Cain found himself the solitary survivor of a universe that was no more than shards of glass.
"Have you ever felt like there’s a world just beyond ours? Some sort of strange dimension, a light dancing just beyond our fingertips? Well, I’ve touched that ‘sacred’ world, and I know its true face."
History is full of interesting and exotic people who can help to populate your world. They can help illuminate the range of what people think is possible for your world (or you can consider them PCs run by players not in your game).
A cult's theology is binding on the god involved: it is possible for that god to believe (and see, just like Winston Smith in 1984) that other gods are mythical, or jumped-up demons, or failing older spirits, even while those gods actually exist and thrive and act in the cosmos, even while working against him.