Lace may have a point about timing.
I was always under the impression that plagiarism was about directly lifting passages of another's work and using it in your own work. To me, this is more like the lifting of a much broader idea. Examples of the accusation are that in both books, Jesus was not crucified, married Mary Magdelane and raised a family. To me, stopping somebody writing about this would be like Bram Stoker rising from the grave and putting a lawsuit on anybody that wrote about vampires. I'm not convinced there's a case to answer here....but then again, that's probably why I'm a mere zug and not some legal eagle