The archaeologist Konstantinos Kourouniotis identified the Mithraeum at Eleusis south of the Sacred House. While the exact date of its construction is not clear, it might relate to details concerning the last hierophant. Historian and sophist Eunapius, writing in the late 4th century CE, recounts an oracle predicting the sanctuary of Demeter’s downfall and the rise of a High Priest not of Athenian citizenship or Eumolpid lineage, traditionally associated with the hierophants, who would worship foreign gods. This last hierophant, interestingly, was a Mithras priest from Thespiae and he may have played a role in establishing the Mithraeum in Eleusis.

The Mithraeum’s existence reflects a time of religious flux and transformation within the Roman Empire, marked by the emergence and popularity of new cults that rivalled traditional deities like Demeter and Persephone. Mithras, an Indo-Iranian god, became particularly revered in early Christian times, especially in the military. Plutarch notes that sailors from Pompey’s fleet were instrumental in introducing this cult.

Mithraism, known for its military and masculine attributes and secretive nature akin to the Demeter cult, spread primarily through Roman legions across Europe, with a stronger presence in the west. Our understanding of its rituals and beliefs derives mainly from the archaeological evidence in mithraea, typically subterranean or cave-like spaces adorned with celestial motifs and centred around the iconic image of Mithras slaying a bull (tauroctony), symbolizing death and rebirth.

The Mithraic mysteries focused on the soul’s journey through celestial realms, corresponding to various initiation levels. The external appearance of mithraea was unassuming, while their interiors featured intricate representations of the Mithraic cosmos.

These worship spaces, accommodating 15-20 people, sometimes up to 40, were intimate in size. The Eleusinian Mithraeum comprised a rectangular chamber with a reused marble Doric propylon from the Peisistratid North Gate at its entrance. Inside, worshippers would recline or sit on platforms along the longer sides to engage in rituals and communal offerings, immersed in the mystical environment of this unique religious sanctuary.