The mysteries were cults of the Greek and Roman world into which one was initiated in secret: the Eleusinian mysteries of Demeter and Persephone, and those of Dionysus, Isis and Mithras. The word mystic comes from mystēs, the initiate. Platonists used the language of the mysteries for philosophical vision.
Mystery religions
Ancient initiatory cults, such as Eleusis, which promised a blessed afterlife through secret rites.
Also: Eleusinian mysteries