Mithraism was at full maturity on its arrival at Rome, the only modifications it ever suffered having been experienced during its younger days in Asia.
Modified though never essentially changed, (1) by contact with the star-worship of the Chaldaeans, who identified Mithras with Shamash, god of the sun,(2) by the indigenous Armenian religion and other local Asiatic faiths and (3) by the Greeks of Asia Minor, who identified Mithras with Helios, and contributed to the success of his cult by equipping it for the first time with artistic representations (the famous Mithras relief originated in the Pergamene school towards the 2nd century B.C.), Mithraism was first transmitted to the Roman world during the 1st century B.C. by the Cilician pirates captured by Pompey.
From the end of the 2nd century the emperors encouraged Mithraism, because of the support which it afforded to the divine right of monarchs.
Finally, philosophy as well as politics contributed to the success of Mithraism, for the outcome of the attempt to recognize in the Graeco-Roman gods only forces of nature was to make the Sun the most important of deities; and it was the Sun with whom Mithras was identified.
The beginning of the downfall of Mithraism dates from A.D.