esoteric

confined to and understandable by only an enlightened inner circle

Designed for, and understood by, the specially initiated alone; not communicated, or not intelligible, to the general body of followers; private; interior; acroamatic; -- said of the private and more recondite instructions and doctrines of philosophers. Opposed to exoteric.

from Greek 'esotericos', inner, secret

“And I understood that, if I was to succeed, certain esoteric knowledge must be acquired, as it were, unofficially.”

— Winston Churchill, Project Gutenberg Complete Works of Winston Churchill

“Gnosis: An Esoteric Tradition of Mystical Visions and Unions.”