The Eleusinian Mysteries (Greek: Ἐλευσίνια Μυστή�?ια) were initiation ceremonies held every year for the cult of Demeter and Persephone based at Eleusis in ancient Greece. Of all the mysteries celebrated in ancient times, these were held to be the ones of greatest importance. These myths and mysteries, begun in the Mycenean period (c. 1500 BC), were a major festival during the Hellenistic era and later spread to Rome.
The rites, cultist worships, and beliefs were kept secret, as initiation was believed to unite the worshipper with the gods and included promises of divine power and rewards in the afterlife. There are many paintings and pieces of pottery that depict various aspects of the Mysteries. Since the Mysteries involved visions and conjuring of an afterlife, some scholars believe that the power and longevity of the Eleusinian Mysteries came from psychedelic agents. _from Wikipedia