Mithras is a mystery cult that appears from the 1st century AD through to the 4th before disappearing. There is no literary source that gives a mythological account of Mithras, and so we are forced to infer from literary mentions of the cult and evidences from monuments. There are some references that refer to a Persian origin, but this too is uncertain. Plutarch suggests Cilicia; Porphyry suggests Zoroaster, but his statement comes quite late (3-4th century).

The cultic myth involves Mithras being born of a rock. He slays a bull and then dines on it with the Sun. Mithraic cult worship generally occurred either in a natural cave, or an artificial one, i.e. the Mithraem which was a temple built to replicate a cave in its structure. The major feature of a Mithraem was a representation of Mithras killing a sacred bull. A second and subsequent scene involves Mithras meeting with the Son, and the two dining together on the bull.

The association with Christianity goes back to Justin Martyr, a Christian author of the 2nd century AD. He actually writes that certain elements of Mithraism imitate Christian practice; it is only in more recent times that attempts to suggest that certain Christian elements originated from Mithraism have appeared. Many of these are simply unfounded assertions. The date of Christmas as Dec 25 is first attested in a 4th century Roman calendar. It also lists a celebration of Sol Invictus at the same time. However there is no clear implication that Sol Invictus involves a celebration of Mithras. Indeed, this is the earliest reference to a feast of Sol Invictus on the same date anyway.

The best internet resource for reading about Mithras and Mithraism is by Roger Pearse. Not only is it well documented, it contains a link to all literary sources and a catalogue of monuments.