I forget which book I read it in, but part of this whole 'mystery' was the mention of the source of Berenger Sauniere's wealth -- contributed by the Vatican to keep Sauniere silent about the bloodline.
According the story, the Prior De Sion, if in fact there was any truth to the 'grail' mystery, the living descendent of Jesus (who was recognized by Pontius Pilate as the 'King of the Jews') would have legal claim even today as King of Israel -- a claim that would supersede the apostolic authority of the Church over Christianity because of the royal blood relationship between Jesus and the current Prior.
Could have been the reason Herod was intent on eliminating any contender to the throne at the time when the order went out to kill all the infants, hoping the kill would have included Jesus, the one he was really looking for because he knew of Jesus' lineage and the bloodline of both parents.
Can't remember the book.
I liked those books, but
I found "The Di Vinci Code"
boring beyond words.