The myth central to the mysteries involves Persephone's abduction by Hades and Demeter's grief, causing winter. Persephone's return each spring symbolizes death and rebirth. Initiates experiencing the mysteries reportedly lost fear of death, gaining conviction about soul's immortality.
The rites remained secret under pain of death; what actually occurred can only be surmised from hints and scholarly reconstruction. Initiates underwent purification, fasted, drank the kykeon (a ritual beverage), and witnessed sacred objects and dramas in the Telesterion.
Whatever transpired, the experience profoundly affected participants. Cicero wrote: "We have gained not only reason to live joyfully, but also to die with better hope." Plato, Aristotle, and Marcus Aurelius were initiates.
The mysteries represent ancient world's most important religious institution outside civic cults. They addressed fundamental human concerns—mortality, meaning, connection to cosmic processes—through direct experience rather than dogma.
Christianity's rise led to their suppression; the sanctuary was destroyed in 396 CE, ending the tradition.
Source
Ancient Greek Religion