TThis is the religion of Mithras who came to the Roman world as an ancient Indo-Perso-Iranian divinity, first as the god of heavenly truth and light and the Lord of all countries. He was the greatest of the Iranian gods, before the coming of Zoroaster in the sixth century BCE. After this he was the sun or the genius of the sun, which was worshipped as a deity by the Persians. In later Zoroastrian theology Mithras was the son of Ahura-Mazda, the god of light and creator of the universe. Mithras, too, was the god of light, of truth, purity, and honor; sometimes he was identified with the sun as leading the cosmic war against the powers of darkness, while always he mediated between his father Ormuzd (Ahura-Mazda) and his followers, protecting them and encouraging them in life's struggle with evil, lies, uncleanliness, and other works of Ahriman, prince of darkness.
When Pompey's soldiers brought this religion from Cappadocia to Europe, a Greek artist pictured Mithras as kneeling on the back of a bull and plunging a sacrificial knife into its neck. This representation became the universal symbol of the faith.
Each seventh day was held sacred to the sun-god; and around the winter solstice his followers celebrated the birthday of Mithras, Sol Invictus, the Invincible Sun, who had won his annual victory over the forces of darkness and would now, day by day, give longer light. (1). Tertullian speaks of a Mithraic priesthood with a high pontiff, celibates, and
virgins serving the god. Daily sacrifice was offered at his altar, worshipers partaking of consecrated bread and wine. The climax of the ceremony was signaled by the sounding of a bell (3). A flame was kept ever burning before the crypt in which the young god was shown felling the bull.
Mithras is commonly represented as a handsome youth, wearing the Phrygian cap, tunic, and cloak, and kneeling on the bull, into whose throat he is plunging a sacrificial knife. The bull is at the same time being attacked by a dog, a serpent, and a scorpion. Also found in mithraea were two small carved figures of men in Persian dress, each holding a torch. Cautes held his torch upright, representing light; Cautopates held his reversed, representing darkness.
Mithraism preached a high morality, and pledged its soldiers to a lifelong war against evil in every form. After death, said Mithraic priests, all men must appear before the judgment seat of Mithras. Unclean souls would be handed over to Ahriman for eternal torment, while the pure would rise through seven spheres, shedding some mortal aspect at each stage, until being received into the full radiance of heaven by Ahura-Mazda himself (3).
This belief spread from India through Persia and the Hellenic world.
Mithraism was popular with Roman soldiers and through them reached Rome in the second half of the 1st Century BCE. By the 3rd - 4th century CE,
soldiers of the Roman Empire had carried it as far west as Spain, Britain, and Germany. Mithraism was a mystery faith in that its rites and doctrines were revealed solely to its followers.
This vigorous religion built its chapels as far north as Hadrian's Wall.
Christian fathers were shocked to find so many parallels between their own religion and Mithraism; they argued that these were thefts from Christianity, or confusing stratagems of Satan (a form of Ahriman). It is difficult to say which faith borrowed from the other; perhaps both absorbed ideas current in the religious air of the East (4).
There is some controversy as to whether the belief system was a religion or in fact more of a brotherhood. A professor of antiquities at Yale has
written a treatise on this subject and he has promised Nova Roma a copy, with permission to publish it here. When and if it arrives, a follow-up
article on that view of Mithras and his following will be developed and published..
Mithraism was an exclusive belief which applied to men only, and seemed to be favored by both soldiers and merchant traders.

The details of the mythology of this belief system were complex, even convoluted. However it is clear that Ahura-Mazda was assisted and supported in His constant struggle against the evil Ahriman, who seems to be equivalent to the modern Christian view of Satan. This struggle appears to be an eternal one pitting the light and goodness against all
that was darkness and evil. This was the central theme of Zoroastrian beliefs.
Mithras was given the task by Ahura-Mazda to go to Earth and hunt for the divine bull. He was to kill this creature and spill its blood, from which all living things would arise.
Thus the most important Mithraic ceremony was the sacrifice of a bull, an event which was associated with the creation of the world. Mithraic ceremonies were held by torchlight in caverns, or in temples converted to resemble such an environment, because Mithras was said to have slain the divine bull in a cave. Tauroctony, finding and slaying the divine bull, was the central theme of Mithraism and formed a significant part of the stone relief carvings found in Mithraic places of worship. In some mithraea, altars to Sol Invictus are also found.
This religion also featured a kind of baptism called the "taurobolium" (throwing of the bull). Initiates stood naked in a ditch below a live bull; when the bull was killed, its blood drenched them, purifying them and giving them a new spiritual and eternal life (5). Some historians contest this version of the ritual, believing instead that the baptism was
conducted with a more restrained but nevertheless serious and meaningful ceremony involving in some way the blood of a bull.
The Brotherhood was divided into seven ascending grades each having its own initiation, baptism, and ritual meal.
There was a form of Mithraism in which the old Persian ceremonies were given a Platonic interpretation. During the 2nd and 3rd centuries CE,
Mithras was honored as the patron of loyalty to the Emperor. After Constantine I accepted Christianity in the early 4th century, Mithraism was suppressed and rapidly declined. Nonetheless, in Germany among other sites, many evidences of its former sway can still be found; for example, the monuments at Heddernheim.
As the monuments of this worship were destroyed during the Islamic conquests, knowledge of its doctrines and rites is necessarily vague and uncertain, but it bears some remarkable similarities to Christianity.
There is a reconstructed model of a mithraeum at Yale, and every year in April a study group called the "Mithracon" is conducted over a weekend.
The participants are treated to a tour of the museum, spend time in the library doing research, gather at local eateries for meals, and take part
in discussions throughout the event on the various aspects of Mithraism.

Notes:

1. Dill, Sir S., "Roman Society From Nero to Marcus Aurelus," London, 1911, page 610.
2. Ibid., pages 601, 623.
3. Cumont F., "Oriental Religions In Roman Paganism," Chicago, 1911, Page 158.
4. Durant, W., "The Story of Civilization: Part III - Caesar and Christ, A History of Roman Civilization from Its Beginnings to A.D. 325,"
   Simon and Shuster, New York, 1944, page 524.
5. Guignebert C., "Christianity, Past and Present," Jesus, NY, 1935, page 71.

Additional References:

• L. & R. Adkins, "Handbook of Life in Ancient Rome;" New York - Oxford, 1994; page 290.
• Encyclopaedia Britannica
• Encyclopedia Americana