Mystes
The Eleusinian ceremonies were secret and no one could join unless he had been specially initiated. Furthermore, one who was initiated had to swear never to reveal the rites (and no one ever did). Someone who had taken such an oath was called “mystes” from a Greek word meaning “close-mouthed.” As a result, such secret societies were called “mysteries.” There were a number of such “mystery religions” in ancient Greece, but of them all, the Eleusinian mysteries were the most famous.
Words from the Myths by Isaac Asimov, 1969.
