Mumbo Jumbo.
If you want to know why that religion got to that state, it was a syncretism of many different faiths, with some faked text to legitimize it over all others, and the destruction of abstraction by claiming someone really died when no untouched historical records exist.
This is an excellent book that sadly only focuses on the aspects described in the DaVinci Code movie, but really explains that the death and resurrection is simply a retelling of more ancient tales.
Just like Romeo and Juliet is a retelling of the more ancient Tristan and Isolda tale.