Even if the historical backdrop was 100% true, the movie still doesn't make one lick of sense.
You've got some pile of bones, allegedly Mary Magdalene. You've got some woman, allegedly her descendent. If it can ever be proven that the woman is related to the bones, the entire Christian religion will come to a screeching halt.
Bull Hockey. You still need a leap of faith to believe the woman in the tomb is Mary Magdalene. Even if it is Mary Magdalene, you still need a leap of faith to believe that she had children by Jesus and not some other person. Then you need a further leap of faith to believe that this somehow disproves Jesus was divine.
Even so, The Da Vinci Code could have been fun if it didn't take itself so gosh darn seriously. A couple years back there was a movie called National Treasure with pretty much the same Knights of Templar conspiracy theory angle, but it was a much more enjoyable movie because it didn't drone on about how anyone who wasn't part of the conspiracy was a pathetic dupe.
My husband and I LOVED "National Treasure"!! It's one of our all time favorites. We bought the DVD and watch it every time it comes on the movie channels. We love it precisely because it treats the plot so lightly and it does give some very REAL glimpses of history. To see the inside of Independence Hall, Library of Congress, etc., is just TOOOO COOL.....
Both the writer and the director of the DeVinci Code wanted their readers/audiences to believe that the story is really real. Overly earnest movies like "The DeVinci Code" and "Left Behind" remind me of Mormon missionary movies.