Well you could say that people's beliefs and power in said belief gives mythological beings strength. Like...ah, gods for instance. Gods didn't exist until humans conceived them in the creation myths, correct? However gods in my world existed before said creations myths but had no way of being brought into our plane of existence. Until, that is, people began to believe in them. So they were sort of trapped and withering away essentially.

[ He pauses before continuing. ]

Naturally the gods, Greek, Roman, Catholic, Japanese all of them put their own twists on the creation myths for themselves and their respective religion. They then put said myths into certain people's heads as ideas, and then once people started preaching them and believing in them they gained enough power to come into our world and feed on the magical energy stored on Earth. And the gods obviously helped their worshipers; so in a way it's a mutual relationship of sorts. A gift from them would be magic! Before gods magic was used poorly, or not as effective as it could have been. Once the gods, and mythological beings, began teaching people it was sort of an...unbreakable relationship really.