Eleusinian

[ el-yoo-sin-ee-uh n ]

/ ˌɛl yʊˈsɪn i ən /

of or relating to Eleusis or to the Eleusinian mysteries.

a native or inhabitant of Eleusis.

Origin of Eleusinian

1635–45; < Latin Eleusini(us) < Greek Eleusínios of Eleusis + -an

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

The Orphic, chiefly deserving mention as the probable foundation of the Eleusinian.
Mysticism and its Results|John Delafield
The thousands that flocked annually to worship the Eleusinian Ceres ceased their pilgrimage.
Willis the Pilot|Johanna Spyri
I greatly fear that I have come across a monument relative to the Eleusinian mysteries.
The King of the Mountains|Edmond About
It is to these last, and not to the Eleusinian mysteries, that the severe strictures of the fathers of the church apply.
Secret Societies of the Middle Ages|Thomas Keightley

The original is simply "after being initiated;" but the Eleusinian mysteries are meant.
Plutarch's Lives, Volume II|Aubrey Stewart & George Long

adj.

1640s, "pertaining to Eleusis," town outside Athens, site of the mystery associated with the cult of Demeter, goddess of harvests, and her daughter.