Mithraism was a cult-like religion that worshiped the sun god of Persian origin named Mithras. The cult was for men only, and involved secret initiation rituals in small, cave-like structures. The cult managed to keep most of its secrets, so is little documented and poorly understood. The religion was introduced to Rome largely by legions returning from Asia Minor after the campaigns of Pompey. The pirates that Pompey suppressed in 67 AD practised Mithraeism. The Emperor Commodus was initiated into the cult in the late second century, and was involved in the religious initiation of Julian the Apostate in the fourth century.

Mithras was a god hero who embodied light and truth. He was known to his followers as the "lord of the wide pastures", and his central action was the capture of a wild bull, which he dragged to a cave and then slaughtered. From its blood sprang life and grain.

    

Mithras, sacrificing a bull, from the 1st century AD, in the museum in Ostia Antica
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Marble Mithraic Relief, with substantial remains of original color and gilding, illustrating the slaughtering of the bull by the god Mithras, from the 3rd century AD, in Hall IX of the National Museum of Rome, Terme di Diocleziano (Baths of Diocletian)
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Several mithraeums can be seen in and around Rome, such as the one in the underground of the Basilica of San Clemente, a few in Ostia Antica, and the Mithraeum Thermarum Antonianarum, underneath the Baths of Caracalla.

    

Mithraeum next to the Baths of Mithras, in Ostia Antica (HDR of 3 images)
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Mithraeum of the Serpents, in Ostia Antica
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Mithraeum of the Seven Spheres, in Ostia Antica
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Mithraic Temple in the underground of the Church of San Clemente
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Mithraism was outlawed in Rome when Christianity became the official religion of the empire in the 380s AD.