What exactly is The Origin of the Brunists properly speaking? Well, for one thing, the accident in the mineshaft caused it on a technical level. Then, in another way, the accidence of Bruno's survival plays a vital role in the formation of a cult, because he is one solitary witness of that horrible deathly moment. Then, there is Eleanor and her catalytic spirit, Domiron. But ultimately, where does the cult itself come from? Its origin is clearly the nature of human psychology.
This is evident in the fact that Bruno didn't do anything to become a famous cult leader. All he did was go in for work at his difficult job (coal mining), and survive an explosion (not by his own doing), and then when he woke up from his coma (like a mini-resurrection), the cult is all but formed already. The portrait is an insightful portrait of the instinctual human religion, which is to celebrate human life and survival by pretending humans are divine.
But for Eleanor that isn't pretend. Domiron shows her things about human nature that other people neglect. That is because, as a crazy person, her point of view has automatic ethos to herself, because she truly experiences her psyche in strange spiritual ways. So she has enough grit to stake her claims of faith. Just like John the Baptist paved the way for Jesus's ministry, Eleanor paves the way for the community to celebrate a day-laborer (like Jesus, the carpenter). Eleanor is a portrait of the human instincts about religious cult behavior.
