One thing that I found strange in the movie is that there was supposedly only ONE living descendant of Christ. Supposing (within the logic of the fiction) that Mary Magdalene did give birth to a child. If that child's lineage was fertile enough to have a descendant after 2000 years, isn't it more likely than not that there would be more lines than just the one?
The only smidgen of logic I saw is that perhaps Brown is saying that the secret order had been killing off all these descendants for 2000 years--in which case you still have the illogic that they didn't get that one line.
Yeah, after 2000 years you would think one of the couples down the line would have more than just one offspring.
Hypothetically, imagine finding out you are directly related to Jesus. Then the secret is exposed to the world. You would have "long lost distant" cousins, 2nd cousins, aunts and uncles, and old college buddies whom you don't quite remember, coming out of the woodwork to ask for favors.