Ancient Iranian religion based on the worship of Mithra , the greatest of Iranian deities before the coming of Zoroaster in the 6th century BC.
It spread from India through Persia and the Hellenic world; in the 3rd–4th century AD, soldiers of the Roman empire carried it as far west as Spain, Britain, and Germany. The most important Mithraic ceremony was the sacrifice of the bull, an event associated with the creation of the world. Mithraic ceremonies were held by torchlight in subterranean caverns. A form of Mithraism in which the old Persian ceremonies were given a Platonic interpretation was popular in the 2nd–3rd century AD in the Roman empire, where Mithra was honored as the patron of loyalty to the emperor. After Constantine I accepted Christianity in the early 4th century, Mithraism rapidly declined.