Mithra is an ancient Indo-Iranian god traceable to the 2nd millennium BCE.
In India, the first surviving recorded instance of Mitra appears in 1400 BCE.
Mitra, in the form mi-it-ra-, is in the inscribed peace treaty of c. 1400 BC between Hittites and the Hurrian kingdom of the Mitanni in the area southeast of Lake Van in Asia Minor. Mitra appears there together with four other Indic divinities as witnesses and keepers of the pact.¹
For Persians, Mithras was a known deity but some grumbled that he didn’t appear in the Gathas (Avestan hymns regarded as sacred scripture) often enough, which made them wonder if the prophet Zoroaster had rejected him in favor of Ahura Mazda, regarded as the supremely wise creator god.
Despite his scarcity in the Gathas, the cult of Mithras was important in Persia. It reached Rome in 67 BCE and become prominent during the Roman Empire as a kind of mystery cult where initiates undertook seven stages of initiation.
Practices included bull sacrifices. One account tells of priests standing underneath a slain bull, literally covered with the animal’s blood. This tradition is abundantly depicted in ancient art, and it is often regarded by learned Christians as a pagan precursor to Christian beliefs about the saving sacrifice of Christ.
Wikipedia sums how little we know about these mysteries. The first quote is from Wikipedia 2010, the time of my previous entry. The second is from Wikipedia 2018:
The most characteristic of these are depictions of Mithras as being born from a rock, and as sacrificing a bull. His worshippers had a complex system of seven grades of initiation, with ritual meals. They met in underground temples, which survive in large numbers. Little else is known for certain.²
Worshippers of Mithras had a complex system of seven grades of initiation and communal ritual meals. Initiates called themselves syndexioi, those “united by the handshake”. They met in underground temples, called mithraea (singular mithraeum), which survive in large numbers. The cult appears to have had its centre in Rome.³
Today scholars debate just how true the Roman version of the mystery cult was to the Persian (i.e. Zoroastrian) practice. It appears the Romans added their own distinct imagery to their mystery religion, which at one time was seen as an alternative or competitor to early Christianity.4
³ https://en.wikipedia.org/wiki/Mithraism (2018/9/23)
4 Vast collection of images at Pinterest: https://www.pinterest.ca/MythicistMKE/mithras/?lp=true