It is quite common for people to claim that the story of Jesus is based on the ancient mystery religions such as that devoted to Mithras, Osiris-Isis etc. The best way to respond to such claims is to look at what the mystery religions actually say. The best resource I have found is Walter Burkert’s Ancient Mystery Cults. This is not an apologetic book and I do not even know if Burkert is a Christian. What I do know is that Burkert is able to present the basics of mystery religions in an understandable way. He dispels myths such as the idea that Mithraism almost surpassed Christianity or that communion had its origins in the mysteries. Burkert’s only agenda seems to be to let the mystery religions speak for themselves and yet in doing that, he demolishes many of the claims of authors such as Tom Harpur, Peter Gandy, Timothy Freke etc. If you want to understand mystery religions and how Christianity is related, this is the book to read.