Everything about the Bene Gesserit sisterhood is designed to intrigue the reader. They exist in a realistic universe, confined by the same laws of physics and reasoning as our own. But through sheer will and discipline, they've managed to bend those rules, gaining seemingly supernatural abilities in the process.
They've been called psychics, priestesses, witches, and mediums, but none of those things are true. They are the great manipulators, masters of the human mind, the overseers of society, always close by but never fully understood.
They kept their mysteries cloaked in rituals and ancient encrypted texts. We knew very little about where they came from. We had heard rumors of a matriarchal society that had sprung up on old Terra countless eons before the events of Frank Herbert's Dune. There were contradictory legends of a group of sorceresses, who had come from another planet filled with poisonous jungles. The nature of their reverend mothers, leaders within the sect, was also a mystery. Some said they were mutants, carrying a specific gene that allowed them to dip into an enhanced state of mind. But in the original novel, Paul Atreides and his mother Jessica were able to attain this state after learning how to subdue a poison that originated from Arrakis. Some of these myths can be decoded and thrown out, but the sisters will always have their secrets, even from the audience.