"The Eleusinian Mysteries, held each year at Eleusis, Greece, fourteen miles northwest of Athens, were so important to the Greeks that, until the arrival of the Romans, The Sacred Way (the road from Athens to Eleusis) was the only road, not a goat path, in all of central Greece. The mysteries celebrated the story of Demeter and Persephone but, as the initiated were sworn to secrecy on pain of death as to the details of the ritual, we do not know what form this celebration took. We do know, though, that those who participated in the mysteries were forever changed for the better and that they no longer feared death." Source: Ancient History Encyclopedia.
"There was an important theater of Dionysus [at Eleusis], and the sanctuary of Demeter and Kore [Persephone] was the site of many festivals of local or national importance ... but the fame of Eleusis was due primarily to the annual festival of the Mysteries, which attracted initiates from the entire Greek-speaking world." Source: Oxford Classical Dictionary (OCD, 3rd ed., revised).
The Eleusian Mysteries have attracted a lot of attention from "psychonauts" like Terrance McKenna and ethnomycologists like Robert Gordon Wasson who believed that initiates may have been given some kind of psychedelic plant as part of their celebrations. Their theory is that the initiates were given an ergot-based hallucinogen, perhaps in liquid form:
"The separation of the hallucinogenic agents [found in the fungi growing on certain species of grass and cultivated plants] by simple water solution from the non-soluble ergotamine and ergotoxine alkaloids was well within the range of possibilities open to Early Man in Greece." -- The Road To Eluesis: Unveiling the Secret of the Mysteries, by R. Gordon Wasson, Albert Hofmann, and Carl A.P. Ruck [p. 43]
Note: I'm not endorsing either Wasson or McKenna, a couple of slippery characters with a lot of suspicious connections, but the idea that hallucinogenic plants of some kind were used in the Eluesian rites isn't all that outlandish.
The Romans helped to preserve and protect Eleusis over the centuries. "Destroyed by the Costobocs in AD 170, [the sanctuary] was rebuilt under Marcus Aurelius, who also brought to completion the splendid propylaea, a copy of the Propylaea on the Athenian Acropolis. In this he followed the initiative of Hadrian, who was primarily responsible for the physical renewal of the sanctuary in the 2nd cent. The sanctuary evidently ceased to exist after AD 395." (OCD) Note: This was about 15 years after the Edict of Thessalonica made Nicene Christianity the official state religion of the Roman Empire.