Da Vinci Code- Spoiler Alert
The Da Vinci Code is a somewhat complex book. It seems to be based on historical facts.
The two major factions are a group called Opus Dei that is linked with the Catholic Church and another group called the Priory of Sion.
Opus Dei is presented as the "bad guys" and the Priory of Sion is presented as the "good guys". Both groups have secret aspects to them.
The Priory of Sion is presented as having been operating for centuries with knowledge of the "true bloodline" of Christ. They also may have access to "proof" that would show that Jesus had children and his bloodline remains to today.
The C's address this issue. They say that Jesus was the father to three children and there are around 340,000 descendents of Jesus alive today. If there are many descendents or just a few, this could be seen as a large threat to Christian religions because the church and "official" sources of religion may not be the only source of legitimate information. In the book Da Vinci Code, it is presented that there may only be three descendents alive today- a man and a woman who are brother and sister plus their mother.
The book focuses a lot of the suppression over time of this direct bloodline. The bloodline had to remain hidden because there were opposing groups trying to wipe them out. Now, this information can be twisted to serve Illuminati interests if they say that they have the true bloodline, and they are in control of the person they put forward as the heir.
I do not think that Dan Brown is forwarding Illuminati interests. He strikes me as a truth seeker. Of course, the information that he uncovers could be used in negative ways. A key point that he makes is that there are secret factions related to the Catholic Church that have been systematically suppressing truth for many centuries. These factions can be seen as part of the Illuminati, so the book can be seen as exposing Illuminati agendas and tactics.
Another major theme of the book is exposing the suppression of the feminine over time. One aspect of this is the suppression of information related to the women in the life of Jesus and the fact that he was a father.