Final answer:
Mystery cults were religious cults that featured secret rituals and became popular in Hellenistic cities. These cults allowed individuals to worship a specific deity and often performed secret initiation rites. The cult devoted to the god Mithras was popular among Roman soldiers.
Step-by-step explanation:
Mystery cults were religious cults that featured secret rituals and became popular in Hellenistic cities. These cults allowed individuals to become initiated in the worship of a specific deity and were devoted to a single god or goddess. They often carried out secret initiation rites and practices, and the cult devoted to the god Mithras was particularly popular among Roman soldiers.