"Mithras [was] an ancient Indo-Iranian god adopted in the Roman empire as the principal deity of a mystery cult which flourished in the 2nd and 3rd cents. AD. Iranian Mithra was a god of compact (the literal meaning of his name), cattle-herding, and the dawn light, aspects of which survive (or were re-created) in his western manifestation, since Roman Mithras was a sun-god (... 'invincible sun god Mithras), a 'bull-killer', and "cattle-thief,' and the savior of the sworn brothers of his cult." -- Oxford Classical Dictionary (3rd ed. revised).
Mithraism and Christianity have a lot in common: the same birth dates, for instance; the occurrence of water miracles, the sign of the cross, etc. According to Roger Pearse, "[t]he idea of a relationship between early Christianity and the cult of Mithras is based on a remark in the 2nd century Christian writer Justin Martyr, who accused the initiates of Mithras of imitating the Christian communion rite." In reality, it was probably the other way around. It seems more likely that the early Christian writers were imitating Mithraism in their culture war against paganism.