The Hymn to Demeter is considered our earliest testament to the existence of a "mystery cult" known as the Eleusinian Mysteries which existed in Greece possibly as early as the Mycenean Age (c. 14th cent. BCE) and certainly by the 8th century BCE. The cult was named after the town of Eleusis, 14 miles west of Athens. The exact relationship between the poem and the celebration of the Eleusinian mysteries remains a matter of speculation.
The Eleusinian mysteries was the most important of the Greek mystery cults: at one period, it seems that a majority of Athenians were initiates. It was also very democratic in that anyone could become an initiate, including those without power in that society: women, the lower classes, and slaves.
The ceremony of the mysteries took place before the fall harvest. It began with a period of fasting and purification. Each participant bathed in the sea together with a piglet, which he or she later sacrificed at Eleusis. Following this, those seeking initiation - referred to as the mystai or "those who close their eyes" - took part in a procession along the Sacred Way from Athens to Eleusis. They were each accompanied by a sponsor who had already undergone initiation rites; initiates were known as the epoptai or "those who see." In the procession, participants danced and shouted rhythmically while swinging bundles of branches. When the procession reached the bridge between Athens and Eleusis, a group of masked figures would ritually mock those on their way to initiation by making obscene gestures and shouting insults.
The actual initiation rite was kept secret and known only to the initiates. It possibly involved the pronouncing of a password, drinking of kykeon (a drink of barley and honey), and the display of agricultural tools and/or an ear of corn. All this took place in a dark hall, until the priest of Demeter appeared and torches were lit. Some have suggested that the abduction of Persephone and mourning of Demeter may have been re-enacted then, in the style of a mystery play. The priest then possibly announced the birth of a divine son. This was followed by dancing and sacrifice.
Like agricultural rituals in other cultures, the mysteries equated rebirth in nature with possible rebirth for human beings. Practical benefits of the rites were believed to include better crops; and more importantly, participation in the mysteries was thought to guarantee a better life after death (what kind of afterlife has remained unclear). The initiation process was thought to mitigate somehow the experience of death, and to suggest that death is not an evil but is something good. Sophocles said about the initiates: "Thrice blessed are those mortals who have seen these rites and thus enter into Hades: for them alone there is life, for the others all is misery."
At the least the mysteries promised the participant a better fate, perhaps a changed, heightened consciousness. One initiate, Sopratos the rhetorician, said: "I came out of the mystery hall feeling like a stranger to myself." Aristotle, in discussing the mysteries, made a distinction between "learning" and "experiencing:" the point of initiation was not to learn (mathein) anything but to experience (pathein) the mysteries, thereby altering one’s state of mind.