I have heard of Celsus before. He was quoted in a book about the origins of Christianity. I don't think it was by Michael Baigent and Richard Leigh, but someone like them. The jist of the book was that there were a lot of god-man myths around the Mediterranean, in which a divine mad dies and is brought back to life. Celsus listed them. I think Osiris was one. Attis, another iirc. According to the authors of this book, most of these cults had levels of initiation. Each cult's text would have their face value meaning. Once the priests thought you were ready, they would initiate you into the secret meaning, and then there would be an inner meaning after that. The authors hypothesised that the gospels were a Jewish attempt at a god-man cult, but unfortunately the Romans slaughtered all the initiates of the secret meanings during the Roman-Jewish wars. This left only Gentile branches of the religion, who incredibly believed in the literal truth of the gospels.
It was quite an entertaining hypothesis. I read quite a few books like that. Eventually I came to the conclusion that they were like books about the identity of Jack the Ripper. One theory sounds amazing and conclusive until you read another that sounds equally amazing and conclusive.
Apparently none of Celsus's writings survive except in apologia written by Christian priests who attempted to refute his charges.