The arguments are going around that Jesus Christ was little more than a mash-up of ancient mythical figures. It is true that, as C. S. Lewis has said, that myths–such as those about death and resurrection–often do find their fulfillment in Christianity, in which, in Lewis’s words, “myth became fact.”
But that isn’t what these folks are arguing; rather, they show that they understand paganism no better than they understand Christianity. Their assertions are just flat-out wrong when it comes to the most basic facts about the myths.
For example, they point out that the Egyptian deity Horus was born of a virgin, whereupon they jump to the conclusion that this is where the virgin birth of Christ comes from. But the birth of Horus, from his mother Isis and his slain father Osiris, is so far from a non-sexual conception that the claim is ludicrous, throwing the radical difference between the Christian and the pagan narrative in high relief. To thoroughly disabuse yourself of the Horus/Jesus parallel, read Joseph Abrahamson, Christ Myth Theory: Horus? Born of a Virgin? Not!.