An esoteric doctrine or treatise; esoteric philosophy.
"The ancient manuscript contained an esoteric treatise that only a select few initiates were permitted to read."
confined to and understandable by only an enlightened inner circle
"a compilation of esoteric philosophical theories"
Intended for or likely to be understood by only a small number of people with a specialized knowledge or interest, or an enlightened inner circle.
"The professor's lecture on ancient Mayan cryptography was so esoteric that only the handful of graduate students in his lab could follow it."