The Modern Resurgence of the Mystery Schools and its Antecedents 
David L. Heifetz, Esq.
In classical antiquity, the initiatory mystery schools were one of the primary means of transmission for esoteric and metaphysical concepts and techniques. The “secrets” they passed to their initiates were the secret teachings of the mainstream religions of the day — ideas and concepts that supplemented and embellished the established civic religious beliefs and practices — and were available only to those found worthy, in many cases, the intellectual elite of the various societies in which they flourished.
From what little knowledge has survived about these groups, we know that their structure and content varied considerably. The Cult of Pythagoras, the Eleusinian Mysteries, the Mithraic Mysteries, the Dionysian Mysteries, the Orphic Mysteries, and a variety of others all had specific and unique secrets which were shared only with their initiates. Many had various levels of initiation or grades each with their own secret teachings, for instance, The Eleusinian Mysteries consisted of the Lesser and Greater Mysteries of Eleusis, whereas the Mithraic Mysteries had an elaborate hierarchy with seven grades or degrees of initiation. In their day these various Mystery Schools were both highly regarded by and important to the societies in which they flourished. Their membership was non-exclusive, and there is evidence that it was common practice for a esoterically inclined person of adequate means to travel the Classical world and be initiated in the various Mysteries as a completion of his education.