The Otherworldly must behave as prescribed by his stage until the power becomes part of him. Until the potion is digested, the bearer is vulnerable: the power is already there, but there is no control over it yet.
The path roles are heavy in themselves: the bearer passes through states from which an ordinary person does not return, and must remain himself in each.
The method remained the knowledge of the few for a long time: the church hierarchs were in no hurry to reveal it, and many Otherworldly rose blindly, paying for it with their sanity.