I didn't read the book, but I had heard enough of the debate about the controversial subject matter to know before seeing the movie that the story is based on the theory that Jesus married Mary Magdalene.
Having this information beforehand, there was nothing that was surprising in the unfolding story. I believe most viewers could see the plot leading to the revelation that the bloodline of Jesus survived to the present day and that a group of evil people wanted to destroy both the bloodline and any proof that it existed.
Most viewers will have concluded, before being told in the movie, that the bloodline extends down to one surviving descendent of Jesus, rather than the hundreds of millions of descendants that would be the more probable result of a bloodline surviving for 2,000 years (more on that later).
That conclusion leads to the conclusion that the sole descendent of Jesus is a main character in the movie, either Robert Langdon, played by Tom Hanks, or Sophie Neveu, played by Audrey Tautou, or possibly both of these characters. One descendant seemed more likely than two and Sophie seemed the more likely of the two, since she had little purpose in the story other than to be beautiful and to be the lone descendant of Jesus.
We receive hints in the movie that Sophie might have healing powers or other residual effects of divine ancestry. I don't understand this coyness by the author. Either she has some divine powers or she doesn't and there's no point in leaving that as a mystery. Apparently the author thought that her lineage would make her a pretty special person whether or not she inherited supernatural powers or divine wisdom.
There are some parallels here to the movie "Dogma," a comedy that attempted to challenge the symbols and rituals of the Catholic Church while ironically taking the symbols and rituals seriously. The main character in that movie, Bethany, played by Linda Fiorentino, was the sole descendent of Mary and Joseph. An angel informed her that Jesus was her (insert several greats here) uncle.
While some movie viewers were asking themselves, "And why is that significant?" Bethany faints, overwhelmed by the enormity of the revelation.
The angel in "Dogma" repeated the generational enumerator "great" several times but nowhere close to 80 times. Using an average of 25 years per generation, there would be 80 generations in 2,000 years, more or less.
In "The Da Vinci Code," there is a secret society determined to destroy the bloodline and another group determined to protect it. Long before 2,000 years elapsed, one group or the other would succeed. The bloodline would either end or expand until it would be impossible to eradicate or even track.
The number of descendants a person has over many generations cannot be precisely predicted. In order to understand multi-generational expansion, we can look at it going the other way. How many ancestors does a person have 80 generations back?
A person has two parents, four grandparents, eight grandparents, etc. We get to over a billion ancestors 30 generations back, which was more people than lived on the planet then. Lines would converge long before 30 generations. They would converge billions of times 80 generations back.
Since racial characteristics were fairly consistent in specific localities in past centuries, most lines of ancestry would obviously converge repeatedly in semi-isolated, culturally homogenous locations. But cultures were not totally isolated. Caucasians (as Sophie Neveu was portrayed) can assume their ancestors 80 generations back included about everybody in most of Europe whose descendants survived to the present.
There were also non-Europeans contributing to the mix through such historic activities as the Moors' conquest of the Iberian peninsula and immigration into Europe of Jews and of Christians with Semitic ancestry.
The probability of someone's bloodline surviving 80 generations, more or less, and being represented by a sole survivor is statistically absurd, not just like winning the lottery but like winning the lottery every week for several weeks as the result of random chance.
But then we don't need to accept random chance when discussing theology. If God wanted there to be only one descendant of Jesus, he could make that happen. This gets us into the theological debate between belief in free will and belief in divine planning.
If having a sole descendant of Jesus on the planet at the start of the 21st Century was predetermined, that removes the drama of the story. That would mean Sophie Neveu's attempt to escape from those determined to kill her was destined to succeed. God guided her feet as she fled for her life and God interfered with the aim of the gunman whose bullet missed her by inches. The thousands of years of human decision making by and about the descendants of Jesus were not implausible, they were just pointless.
Aside from theological questions, the movie is a mediocre action thriller. A lot of high caliber acting talent and high budget, on-location filming were thrown at a weak screenplay, creating a disappointing result. An understanding of basic mathematics also contributes to disappointment in this movie.