http://eawc.evansville.edu/essays/mithraism.htmMithras is the Roman name for the Indo-Iranian god Mitra, or Mithra, as he was called by the Persians. Mitra is part of the Hindu pantheon, and Mithra is one of several yazatas (minor deities) under Ahura-Mazda in the Zoroastrian pantheon. Mithra is the god of the airy light between heaven and earth, but he is also associated with the light of the sun, and with contracts and mediation. Neither in Hinduism nor in Zoroastrianism did Mitra/Mithra have his own cult. Mitra is mentioned in the Hindu Vedas, while Mithra is is the subject of Yashts (hymns) in the Zoroastrian Avesta, a text compiled during the Sassanian period (224-640 CE) to preserve a much older oral tradition......The archaeological evidence for Mithraism, consisting mostly of monuments, inscribed dedications, and the remains of mithraea, indicates that the cult was most popular among the legions stationed in frontier areas. The Danube and Rhine river frontier has the highest concentration of evidence, but a significant quantity of evidence amply demonstrates that Mithraism was also popular among the troops stationed in the province of Numidia in North Africa and along Hadrian's wall in England. The inscriptions on dedications found in all these areas support Cumont's assertion that Mithraism was most popular among legionaries (of all ranks), and the members of the more marginal social groups who were not Roman citizens: freedmen, slaves, and merchants from various provinces (see above).
The area where the concentration of evidence for Mithraism is the most dense is the capital, Rome, and her port city, Ostia. There are eight extant mithraea in Rome of as many as seven hundred (Coarelli 1979) and eighteen in Ostia. In addition to the actual mithraea, there are approximately three hundred other mithraic monuments from Rome and about one hundred from Ostia. This body of evidence reveals that Mithraism in Rome and Ostia originally appealed to the same social strata as it did in the frontier regions. The evidence also indicates that at least some inhabitants knew about Mithraism as early as the late first century CE, but that the cult did not enjoy a wide membership in either location until the middle of the second century CE.