Mithraism is the best-attested Roman cult about whose beliefs we know almost nothing, and the gap between those two facts is the whole subject.
What survives is abundant and physical. Several hundred mithraea have been excavated across the empire, from Hadrian's Wall to the Euphrates. They are consistent: a small windowless room, usually sunk below ground or built to feel so, with raised benches down each side for reclining at a meal, and at the end the tauroctony, Mithras in a Phrygian cap kneeling on a bull and driving a knife into its neck, while a dog and a snake go to the wound, a scorpion attacks the bull's genitals, a raven watches, and two torchbearers stand with torches raised and lowered. Zodiac signs frequently surround the scene. Inscriptions name members, their seven grades (Corax, Nymphus, Miles, Leo, Perses, Heliodromus, Pater), and their occupations: soldiers, officials, freedmen, merchants. Membership was exclusively male.
What does not survive is anything from inside. There is no Mithraic scripture, no liturgy, no prayer, no theological text, and no account of the cult written by a member. Not a line. What we have is iconography without a caption, and Christian polemicists describing a rival.
This has two consequences that are routinely ignored.
First, the tauroctony has no agreed meaning. It plainly means something specific, it is too standardised not to, but reconstructions range from an astronomical code, in which each figure maps to a constellation and the scene records a celestial event, to a soteriological drama of sacrifice and renewal. Roger Beck's astronomical reading is the most developed and is not universally accepted. Anyone who tells you what the bull-killing signified is offering a hypothesis.
Second, the Persian origin story is largely dead. Franz Cumont argued in the 1890s that the cult was Zoroastrian Mithra-worship carried west by soldiers and slaves, and that framework dominated for decades. From the 1970s it came apart: the Iranian Mithra has no bull-killing, the grade structure and the mithraeum have no Iranian precedent, and the cult appears in the Roman world abruptly in the late first century AD rather than arriving gradually along a traceable route. The current consensus treats Roman Mithraism as a Roman creation using Iranian dress (the name, the cap, the eastern flavour), for a religion invented in and for the empire.
The parallels with early Christianity that circulate online (a 25 December birth, twelve disciples, resurrection), are not attested in the Mithraic evidence and derive from nineteenth-century comparative works and from later popular writing. They should not be repeated.