Let me start by saying that I understand that the book is historically inaccurate (to put it mildly) and makes absurd claims. Let me also make the disclaimer that I have not read it.
However, the book is popular, and it is popular among some Christians. Why?
The Da Vinci Code portrays Jesus as a human being. I think any portrayal of Jesus humanity has the potential to be popular. Jesus was, of course, a human being. He was also God. I think Jesus divine nature is over-emphasized today, especially in Catholicism, but also among Protestants. The Da Vinci Code is feeding a yearning people have to know the whole picture of Jesus. Unfortunately they are filling that need with fiction.
To illustrate the point that Jesus’s divine nature might be over-emphasized, I will ask a question:
Have you ever pictured Jesus being friends with the Apostles, or with anyone for that matter? If the very idea seems strange, I think maybe you over-emphasize Jesus’ divinity.
“The Passion of the Christ” carried on the tradition of over-emphasizing Jesus divine nature. Every act of Jesus was accompanied by dramatic music and slow motion hollywood effects.
Even the one scene “The Passion” that had the potential to give us a peek at Jesus humanity over-emphasized Jesus divinity. Jesus was doing his job as a carpenter, also accompanied by dramatic music, slow motion effects, and serious looks between the actors that played Jesus and Mary. Jesus even innovated a modern table and chair. I found that scene absurd.
All-in-all, Mel Gibson used every trick in the Hollywood playbook to manipulate the emotions of the audience into feelings of awe. Because I was interested in a glimpse of how Jesus really lived, I was dissapointed.