The Eleusinian Mysteries were the most prestigious initiatory cult of the ancient world, and their central secret was kept so well that it is still not fully known.
Demeter, Persephone and the grain
The mysteries re-enacted the myth of Demeter, goddess of grain, and her daughter Persephone, seized by the lord of the dead and returned for part of each year, an account of the seasons and of death and renewal. Eleusis, on the coast west of Athens, was where the grieving Demeter was said to have rested and where she established her rites. The sanctuary was built around the Telesterion, a great pillared hall in which the culminating revelation took place.
Initiation and the oath of silence
Initiation was open to almost anyone who spoke Greek and was free of blood-guilt, women and enslaved people included, which was unusual. Candidates walked in procession from Athens along the Sacred Way, fasted, drank a barley preparation called the kykeon, and were shown things in the darkened Telesterion that promised them a better lot after death. What exactly was shown or said was protected by an absolute oath; to reveal it was a capital matter, and the ancient writers who underwent it kept faith. Scholars reconstruct only fragments: things done, things said and things shown.
Emperors, philosophers and the end
For close to two thousand years the mysteries drew participants from across the Greek and Roman world, including several emperors and philosophers who valued them as the highest of the traditional cults. They ended when the sanctuary was sacked in the late fourth century CE and the empire's new Christian order suppressed pagan worship. The site is an archaeological reserve outside modern Elefsina.