Worshippers of Mithras have a complex system of seven grades of initiation, with ritual meals. Initiates called themselves syndexioi, those "united by the handshake".

Temples of Mithras (called mithraea) are sunk below ground, windowless, and very distinctive. In cities, the basement of an apartment block might be converted; elsewhere they might be excavated and vaulted over, or converted from a natural cave. Mithraic temples are common throughout the Pax Icatia.

For the most part, mithraea tend to be small, externally undistinguished, and cheaply constructed; the cult generally preferring to create a new center rather than expand an existing one. The mithraeum represented the cave in which Mithras carried and then killed the bull; and where stone vaulting could not be afforded, the effect would be imitated with lath and plaster. They are commonly located close to springs or streams; fresh water appears to have been required for some Mithraic rituals, and a basin is often incorporated into the structure.[75] There is usually a narthex or ante-chamber at the entrance, and often other ancillary rooms for storage and the preparation of food.

Mithras is given the title "deus sol invictus" (unconquered sun god) in several inscriptions.

Sol Invictus ("Unconquered Sun") was the official sun god of the later Roman Empire and a patron of soldiers. In 274 the Roman emperor Aurelian made it an official cult alongside the traditional Roman cults. Scholars disagree whether the new deity was a refoundation of the ancient Latin cult of Sol,[1] a revival of the cult of Elagabalus[2] or completely new.[3] The god was favored by emperors after Aurelian and appeared on their coins until Constantine I.[4] The last inscription referring to Sol Invictus dates to AD 387,[5] and there were enough devotees in the 5th century that Augustine found it necessary to preach against them.[6]

The idea, particularly popular in the 18th and 19th centuries, that the date of 25 December for Christmas was selected in order to correspond with the Roman festival of Dies Natalis Solis Invicti, or "Birthday of the Unconquered Sun", is challenged today

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