In the Roman Empire, from the first to the fourth century CE, a distinct mystery cult of a god called Mithras spread widely, especially among soldiers, meeting in cave-like underground mithraea and centred on the image of the god slaying a bull (the tauroctony). The relationship between this Roman Mithras and the Iranian Mithra is one of the most debated questions in the history of religions: the Romans themselves believed the cult was Persian, and the name is plainly the same, but the central bull-slaying myth and the astrological symbolism have no clear Iranian precedent, and many scholars now see the Roman cult as a substantially new creation that borrowed an eastern name and prestige. The two should therefore be kept distinct, however they are related. The Iranian Mithra, meanwhile, continued, and continues, as a yazata of the living Zoroastrian tradition.
The Iranian-origin theory itself has a datable author: the Belgian scholar Franz Cumont set it out in a two-volume study published in 1896 and 1899, arguing that Roman Mithraism was essentially a westward transplant of Zoroastrian religion, and his reconstruction dominated the field for the first half of the twentieth century. It was challenged decisively at the First International Congress of Mithraic Studies, held at Manchester in 1971, where scholars including Richard Gordon argued that Cumont had read the Iranian evidence too imaginatively; a chief objection is that in genuine Iranian sources Mithra is never said to kill a bull, so the tauroctony at the centre of every Roman mithraeum has no real Iranian precedent, however Persian the god's name and the cult's own self-image remained.