What sources do you have for this?
Various sources for various things related.
The Oxford Classical Dictionary says this about Mithraism's transmission.
Did the cult develop from and perpetuate a stream of Zoroastrianism, or was it essentially a western creation with 'Persian' trimmings? There is no agreement because there is so little evidence.
Literary references to Mithras and Mithraism are as scarce as the material remains are abundant.
Circumstantial evidence indicates that Mithraism originated in the Persia and spread from there. As I understand it, most of the literature about Mithraism is by non-Mithraists, but it is quite possible that it changed dramatically from its Persian origin.
2 per cent of the entire population at the very most. Scarcely the great rival to Christianity that inflated views of the cult have sometimes made it. --- OCD, Revised Third Edition, P 991.
I have never seen census records that reported religion from that period, but I have read that Mithraism had become more popular than the Greek and Roman religions in the East and among the military.
You might possibly be selling the early Christians a bit short here or overestimating the influence Mithraism might have had on Christianity.
I don't think so, but no one was providing detailed information along those line, so we can't be sure.
The only way we can discern anything about the Mithraic narrative is from icons, mainly bas-reliefs recovered from Mithraea.
He was born from a rock. He sacrificed a bull. He did lunch with the Sun. That seems to be it although zodiacal relevance and mythological templates can (not necessarily invalidly) be applied to Christ's narrative from some points-of-view and such comparisons can often be illuminating.
I'm very suss about the whole Mithraism informing Christianity thing although I'd more than happy to be shown to be wrong. I do think we need to be careful about how much credence we pay to the likes of Manly Palmer Hall and others who make spectacular claims without quoting sources, who quote sources who don't, who quote completely silly sources or who make it up as they go along.
How much Christianity took from Mithraism is a guess at ths point, and will continue to be a guess until we get a good time machine. But when one looks at the differences between the Gospels and practice and notices correlations with other things, it appears that significant parts of Mithraism were borrowed by the Christians.
