Final answer:
Mithraism was a largely unknown religion during the Roman Empire which offered benefits to its worshippers, particularly popular among soldiers; characterized by secret initiations and focused on the deity Mithras.
Step-by-step explanation:
The religion that was mostly unknown and offered benefits to worshippers during the Roman Empire was Mithraism. Mithraism originated from ancient Persia and was introduced to Rome around the second century CE. It was particularly popular among Roman soldiers and centered on the sacrifice of a sacred bull by Mithras. Despite its popularity, many details about its practices remain mysterious due to the secretive nature of its initiation rites.
In general, mystery cults in the Roman Empire offered esoteric knowledge and exclusive membership benefits. The cults of Mithras, Isis, and others like the Dionysian and Eleusinian Mysteries became popular alongside traditional Roman religious practices. These cults often promised spiritual salvation, well-being, or otherworldly rewards through their secret rituals, creating a close-knit community of initiates. The private, exclusive nature of these cults contrasted with the public, inclusive civic religions in Rome.