In Chapter 1 of Monmusu Quest, it is shown that a large number of humans worshiped Ilias. Yet, not all human communities consider her as a goddess. The worship of Ilias also seems to be the root of the conflict between most humans and monster girls considering how much Alice detested Luka's worship of her. The conflict wasn't always the case either, it seems to have arisen after humans started worship Ilias.
Why did humans started worshiping Ilias? How did it get started?