A man named Pierre Plantard forged documents purporting that Jesus and Mary Magdalene has children whose descendants became the royal family of France. Plantard seeded various archives with these forgeries, then arranged for his cohorts to "discover" them. It was a hoax that forms the supposed "factual" basis for Brown's novel.
All of this is in the service of persuading people that Christian sexual morality is a lie, imposed on us power-mad, killjoy priests.
yes you are correct. he forged the "ancient" documents totally this all is known and admitted. But the author acts like it's true because well, he wants to make a gazillion dollars.