In Greek myth, Dionysus was conceived of human Semele and the god Zeus in the usual way of intercourse. Zeus’s wife Hera, being jealous, tricked Semele into extracting a promise from Zeus to reveal to her all his glory, which burned Semele into a heap of ashes. Zeus took immortal Dionysus out of the heap and sewed him under the skin of his thigh until he was ready to be born. No virgin birth there.
Look up Horus and Mithra in Wikipedia and you will not find the Christ-resembling elements that Peter Joseph attributes to them mentioned - hence not important or consistent parts of their myths, if there is any accuracy to them at all.
C.S. Lewis noted and answered the pagan foreshadowings and echoes of Christ many years ago.
I think the reason that religious gadflys point to Mithras often when trying to undercut Christianity is the whole notion of December 25th being Mithras' birthday.
Mithraism and Christianity were two competing religions during the Roman Empire, and it's possible that they shared, borrowed, stole, etc. different traditions from each other. Christianity won out in the end mostly because of its universality and allowance of women into the fold, whereas Mithraism was more of a male-only secret society.
Since Dec. 25th isn't in the Bible though, I don't see how this alone could shake someone's faith.
Thank you very much -- I was going to post the same thing. Then, they are more than a few wackos who believe that Lewis was a pagan sun-worshipper. Do a Google and your jaw will drop.