The Eleusinian Mysteries10:38
The Eleusinian Mysteries, held annually in honor of Demeter and Persephone, were the most sacred and revered of all the ritual celebrations of ancient Greece. They were instituted in the city of Eleusis, some twenty-two kilometers west of Athens, possibly as far back as the early Mycenaean period, and continued for almost two thousand years. Large crowds of worshippers from all over Greece (and later, from throughout the Roman empire) would gather to make the holy pilgrimage between the two cities and and participate in the secret ceremonies, generally regarded as the high point of Greek religion.
In the Hellenistic age (300-150 BCE), the cult was taken over and run by the state, and two aristocratic families from Eleusis officiated (the Eumolpidae and Kerykes). In this age, mystery cults were becoming very popular, unlike classical Greece (400s BCE) when the Eleusinian mysteries were a rare form of worship. The annual Eleusinian mysteries attracted thousands of people from all over the Greek world, and the only initial requirement to become a mystes (initiate) was to be without blood guilt nor a barbarian ( in other words, if you spoke Greek). It was open to both men and women, and remarkably, slaves were also allowed into the cult.
The mysteries existed from Mycenaean times (circa 1600-1200 BCE), thought to have been established in the 1500s BCE and held annually for two thousand years. The Roman emperor Theodosius closed the sanctuary in CE 392, and finally it was abandoned when Alaric, king of the Goths, invaded Greece in CE 396. This brought Christianity to the region, and all cult worship was forbidden.