 The mysteries of Isis were religious initiation rites performed in the cult of the goddess Isis in the Greco @-@ Roman world . They were modeled on other mystery rites , particularly the Eleusinian Mysteries in honor of the Greek goddess Demeter , and originated sometime between the third century BCE and the second century CE . Despite their mainly Hellenistic origins , the mysteries of Isis were popular in the Roman Empire , and continued to be celebrated throughout the Roman period . The mysteries of Isis were held in a number of cities , including Rome itself . The rites were held in a sanctuary , the mysterium , where the initiate was expected to remain silent