Mithraism is a completely different creature than Christianity.
It's a truly pagan religion only for men with NO written scriptures.
What's to say the fail didn't take up any new idea of the day?
What Mithraism and Christianituy have in common are very superficial and not in actual religious doctrine.
What Christiasn believe we believe comes from Jewish scriptures, not imported from other faiths.
I could point out to you, for instance, how Isaiah 53 talks about the passion, death, and ressurection of Christ.