A mystery was once something you could be executed for discussing. The word comes from Greek mystḗrion, meaning 'secret rite', from mystēs ('one initiated'), from myein — 'to close'. Specifically: to close the lips.
The Eleusinian Mysteries, held annually near Athens for nearly two thousand years, were the most sacred rites in the Greek world. Initiates witnessed secret ceremonies honouring Demeter and Persephone. What exactly happened inside the Telesterion hall at Eleusis remains unknown — because the penalty for revealing it was death. The silence held so well that, despite
Alcibiades, the Athenian general, was accused in 415 BCE of profaning the mysteries at a private dinner party. The scandal contributed to his exile and the unravelling of Athens's Sicilian expedition.
Christian theology adopted mystērium to describe truths beyond human understanding — the mystery of the Trinity, the mystery of the Incarnation. Medieval mystery plays took their name from a different word entirely (Latin ministerium, 'ministry'), but the spelling collision merged the two concepts in popular imagination.
The modern detective mystery — the whodunit — is a gentle echo of the original. Where the Greek mysterion was a secret too sacred to reveal, a modern mystery is a puzzle that begs to be solved.