Mithraism

Mithraism was brought to Europe from its Persian roots after Alexander the Great’s conquests. Extremely popular among Roman soldiers, it became one of the ancient Roman mystery cults, religious sects which were restricted to initiates and were generally quite secretive. Mithras, as he was known to the Romans, was the Persian god of the sun, or at least the airy light between heaven and earth. There is not much surviving text about Mithraism, least of all a central holy book, which may never have existed. Most of what we know about the religion comes from the ruins of its temples. These were commonly located underground and were cheaply constructed, as the followers preferred to make a new temple whenever the old one wore out.
A detail which separates Roman worship of Mithras from the Persian god is that he is often shown slaying a bull, which has led to a lot of confusion among archaeologists. One of the most important dates in their calendar was December 25, which was recognized as Mithras’ birthday. Because of this, and a few other details, some people believe parts of Christianity may have evolved from this religion, although it is very hard to prove.