Esotericism developed in the Middle Ages. Christian esotericism regarded religion as a mystery, available only to a small circle of initiated people.
Esoteric knowledge, in the dictionary (non-scholarly) sense, is thus that which is available only to a narrow circle of "enlightened", "initiated", or specially educated people. Esoteric items may be known as esoterica. In contrast, exoteric knowledge is knowledge that is well-known or public; or perceived as informally canonic in society at large.