Most westerners tend to attribute all the glory of this misteries to some drug, disregarding important elements such as magic and spirituality.
When it comes to these ancient misteries, like the ones from Eleusis and Egypt, even if there was some entheogen involved, that´s not the entire story. There was an specific priesthood around these misteries, they would do rituals or invocations, summmoning a specific spirit or higher entity.
Many drugs can serve as a bridge to higher realms, it´s like a phone call. But you can´t make the same phone call over a period of thousands of years of history, unless you are always dialing to the same number, that is what the ritual represents.
By the way, talking about Eleusis, I wondered what this mistery was about, according to records everyone had the right to attend it in a lifetime and it lasted one day. So... I guess meditation and tantric sex can´t be taught in 24 hours... an entheogen makes more sense... there´s even a talk from Terence McKenna, telling about certain greek citizen who stole the "misteries" and was giving it to friends at his house as entertainment... that would make a good case for ergot or other substance.
There´s a second alternative involving a priesthood with what we could call "superpowers". The type of people who could awake you third eye, or take you astral body out from your physical body (like the superhero movie Dr. Strange). This is not unheard of, there are some gurus in India who are known to charge you 10.000$ for unblocking some of your chakras.
That would be a strong case for an initiation taking place in a short period of 24 hs.
"...after five seconds I was no longer a marxist, no longer a materialist, no longer a rationalist.
It killed those things, it cauterized them..."Terrence McKenna