A similarity between Mithra and Christ struck even early observers, such as Justin, Tertullian, and other Fathers, and in recent times has been urged to prove that Christianity is but an adaptation of Mithraism, or at most the outcome of the same religious ideas and aspirations (e.g. Robertson, "Pagan Christs", 1903). Against this erroneous and unscientific procedure, which is not endorsed by the greatest living authority on Mithraism, the following considerations must be brought forward. (1) Our knowledge regarding Mithraism is very imperfect; some 600 brief inscriptions, mostly dedicatory, some 300 often fragmentary, exiguous, almost identical monuments, a few casual references in the Fathers or Acts of the Martyrs, and a brief polemic against Mithraism which the Armenian Eznig about 450 probably copied from Theodore of Mopsuestia (d. 428) who lived when Mithraism was almost a thing of the past -- these are our only sources, unless we include the Avesta in which Mithra is indeed mentioned, but which cannot be an authority for Roman Mithraism with which Christianity is compared. Our knowledge is mostly ingenious guess-work; of the real inner working of Mithraism and the sense in which it was understood by those who professed it at the advent of Christianity, we know nothing. (2) Some apparent similarities exist; but in a number of details it is quite probable that Mithraism was the borrower from Christianity. Tertullian about 200 could say: "hesterni sumus et omnia vestra implevimus" ("we are but of yesterday, yet your whole world is full of us"). It is not unnatural to suppose that a religion which filled the whole world, should have been copied at least in some details by another religion which was quite popular during the third century. Moreover the resemblances pointed out are superficial and external. Similarity in words and names is nothing; it is the sense that matters. During these centuries Christianity was coining its own technical terms, and naturally took names, terms, and expressions current in that day; and so did Mithraism. But under identical terms each system thought its own thoughts. Mithra is called a mediator; and so is Christ; but Mithra originally only in a cosmogonic or astronomical sense; Christ, being God and man, is by nature the Mediator between God and man. And so in similar instances. Mithraism had a Eucharist, but the idea of a sacred banquet is as old as the human race and existed at all ages and amongst all peoples. Mithra saved the world by sacrificing a bull; Christ by sacrificing Himself. It is hardly possible to conceive a more radical difference than that between Mithra taurochtonos and Christ crucified. Christ was born of a Virgin; there is nothing to prove that the same was believed of Mithra born from the rock. Christ was born in a cave; and Mithraists worshipped in a cave, but Mithra was born under a tree near a river. Much as been made of the presence of adoring shepherds; but their existence on sculptures has not been proven, and considering that man had not yet appeared, it is an anachronism to suppose their presence. (3) Christ was an historical personage, recently born in a well known town of Judea, and crucified under a Roman governor, whose name figured in the ordinary official lists. Mithra was an abstraction, a personification not even of the sun but of the diffused daylight; his incarnation, if such it may be called, was supposed to have happened before the creation of the human race, before all history. The small Mithraic congregations were like masonic lodges for a few and for men only and even those mostly of one class, the military; a religion that excludes the half of the human race bears no comparison to the religion of Christ. Mithraism was all comprehensive and tolerant of every other cult, the Pater Patrum himself was an adept in a number of other religions; Christianity was essential exclusive, condemning every other religion in the world, alone and unique in its majesty.
People forget that most of what we know about the ancient past has passed through Christian hands. Even Greek philosophy is really seen by us through Christians lenses.