Which groups played an integral role in the development of "mystery religions"?

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Greek and Roman groups played an integral role in the development of "mystery religions," which were characterized by secret ceremonies and initiation rites. The Eleusinian Mysteries, for example, were a popular Greek mystery religion that involved worship of the goddess Demeter and her daughter Persephone. The cult of Mithras, a mystery religion from ancient Rome that involved initiation ceremonies and the worship of the god Mithras, also gained popularity in the Roman Empire during the 2nd and 3rd centuries CE.

Which Groups Played an Integral Role in the Development of "Mystery