Mystery religions, sacred Mysteries or simply mysteries, were religious cults

Cult (religious practice)

In traditional usage, the cult of a religion, quite apart from its sacred writings , its theology or myths, or the personal faith of its believers, is the totality of external religious practice and observance, the neglect of which is the definition of impiety. Cult in this primary sense is...

 of the Greco-Roman world

Greco-Roman world

The Greco-Roman world, Greco-Roman culture, or the term Greco-Roman , when used as an adjective, as understood by modern scholars and writers, refers to those geographical regions and countries that culturally were directly, protractedly and intimately influenced by the language, culture,...

, participation in which was reserved to initiates.
The main characterization of this religion is the secrecy associated with the particulars of the initiation and the cult practice, which may not be revealed to outsiders. The most famous mysteries of Greco-Roman antiquity were the Eleusinian Mysteries

Eleusinian Mysteries

The Eleusinian Mysteries were initiation ceremonies held every year for the cult of Demeter and Persephone based at Eleusis in ancient Greece. Of all the mysteries celebrated in ancient times, these were held to be the ones of greatest importance...

, which were of considerable antiquity and predated the Greek Dark Ages

Greek Dark Ages

The Greek Dark Age or Ages also known as Geometric or Homeric Age are terms which have regularly been used to refer to the period of Greek history from the presumed Dorian invasion and end of the Mycenaean Palatial civilization around 1200 BC, to the first signs of the Greek city-states in the 9th...

.