You might want to read the little bit Dan Brown puts in the beginning of his book
He explains it
HERE.
Excerpt from interview:
BUT DOESN'T THE NOVEL'S "FACT" PAGE CLAIM THAT EVERY SINGLE WORD IN THIS NOVEL IS HISTORICAL FACT?
If you read the "FACT" page, you will see it clearly states that the documents, rituals, organization, artwork, and architecture in the novel all exist. The "FACT" page makes no statement whatsoever about any of the ancient theories discussed by fictional characters. Interpreting those ideas is left to the reader.