The cult of Mithras is a particularly useful choice for a comparison with Christianity, especially Roman Catholicism, because several elements of Mithraic religion were adopted by early Christians (the clue is in the name – it’s a Roman religion!). Ancient Mithraists shared a ritual meal of bread and wine, just like early Christians (though the Christian detail of the unleavened bread comes from the Jewish Passover). Both Mithras and Jesus were associated with light and the sun. The early Christians deliberately took over Mithras’ birthday on the 25th December to celebrate the birthday of Jesus instead, a date conveniently close to the Roman winter festival of Saturnalia (17-23 December), though nowhere near where the Christian gospels would place the birth of Jesus (if there were shepherds out all night watching sheep and lambs, it must have been spring). And, like most mystery cults, Mithraism offered personal salvation in this life and the next, just as Christianity did. In 1882, a historian called Ernest Renan actually suggested that if Christianity had not taken over the Roman world, Mithraism would have done – most modern historians would disagree with that, but the association has stuck.
However, some elements of the “Mithraic” religion in the show would be completely unrecognizable to an ancient follower of Mithras. Ancient Mithraism was not a monotheistic religion. The mystery cults included cults to the Egyptian goddess Isis, the Anatolian goddess Cybele, the Greek god Dionysus and the Greek hero Orpheus, and they weren’t rivals to ancient pagan religion in general; they were add-ons. Everyone in the Roman Empire (except Jews) was expected to worship the main state gods – Jupiter, Juno, Neptune and so on – as well as the cult of the emperors who had become gods (i.e. the ones who hadn’t been assassinated).
We call them “mystery cults” because the rituals they practiced, the “mysteries” or “secret rites”, were kept secret from anyone who wasn’t initiated into the cult. They were members-only clubs, which you had to pay to join, and go through an initiation ritual. We don’t know exactly what these were like because, of course, they were a secret! Only members could learn the secrets of the god or goddess and take part in the secret ceremonies. Like modern Freemasonry, ancient Mithraism also allowed members to rise through the ranks of the cult, gaining different levels as they went. Also like Freemasonry, and unlike the other mystery cults, membership was usually restricted to men.
All this means that a lot of the attitudes of the “Mithraic” on the show would sound completely weird to ancient Mithraists. They would be especially confused by the Mithraic reliance on a book of “Scriptures”, since it was forbidden on pain of death to write down anything about the sacred mysteries or the secrets of the cult. Most of what we know about ancient Mithraism, we’ve put together from images and inscriptions from inside the secret chamber of the Mithraeum. This was a place of worship designed to look like a cave, which only people who had been initiated into the cult were allowed to enter, so no one outside the cult would see the images or know the cult’s secrets. The idea that any group would only be allowed to hear stories from “Scriptures” represents an extreme minority even for Christianity, but would have been completely confusing to ancient Mithraist.
Ancient Mithraists were also extremely unlikely to get involved in any kind of holy war. Because the cult was an add-on to the worship of many gods, the members of the cult would be involved in worshipping lots of other gods anyway, with Mithras as an added personal extra. Ancient people often weren’t too keen on atheists, as any refusal to sacrifice to the gods might endanger everyone if the gods got angry about it, so a holy war against atheists might be more likely than one against another religion, but it wouldn’t be because they thought the atheists should all worship their one specific god.