Esotericism. So called secret stuff. According to some the hidden meaning behind all religions. Their origins. Commonly known in ancient times as 'Old religion'. Its present name...'Perennial Philosophy'. Passed down in those same ancient days ‘orally, before the written word came into common use.
To unlock that subject…understand its 'framework'. That same framework that in 'modern' times came to be known as the 'divisions' [i.e.,Greater = ''higher'' / Lesser = ''lower.'' Explained within] - in the so called 'Mystery' clubs…Whether they be known as [examples] - Eleusis / Dionysus / Mithraic / Zoroastrian / Pythagorean / Rosicrucian or Freemasonry. Or that same framework represented within the structure of stone monuments such as the Pyramids / Cathedrals / Churches / Temples etc. Or…the same...represented in some form or other in... literature, music and art.
To unlock that framework. Find those 'keys'. [Keys of Solomon].