The Myst yields up its power under a force of belief. It's not impossible, therefore, for groups of people who believe in something to make it very nearly real. Even in a world without true gods, people are still people and tend to create gods where they are needed- to spark a cause, to give the sense of freedom, to gain power. It's not known if The Created are truly beings in their own right, but they do seem to have influence and power in the world. They come in many forms and because of many things but one thing unites them all: they need faith to survive.