Dan Brown has followed the classic route of a disinformation artist. His first novel, Digital Fortress, which was published in 1998 explores the theme of government surveillance of electronically stored information on the private lives of citizens, and the possible civil liberties and ethical implications of using such technology. Having established his credibility as a concerned citizen Brown then went on to spread disinformation about some of the major themes of the Rothschild secret empire including, in his 2001 novel Deception Point, the notion that governments are hiding the truth about extraterrestrial life.
Dan Brown has written 4 novels on the subject of secret societies and their symbology. Both The Da Vinci Code and Angels and Demons have been made into Hollywood movies, which demonstrates emphatically that the stories are not regarded as a threat by the establishment who control the US film industry. A film adaptation of The Lost Symbol is confirmed to be in production for 2013 release.  What then is the objective of establishment backed stories about secret societies? Now that the internet has allowed the cat out of the bag and people are aware of the Knights Templar; the Rosicrucians; the Freemasons; the Illuminati and the esoteric symbology they share, these organisations are attempting to influence the public's perception through controlled disinformation.
The Da Vinci Code
The 2009 edition of the Da Vinci Code includes an introductory page entitled "Fact" which includes the following information:
"The Priory of Sion - a European secret society founded in 1099 - is a real organisation. In 1975 Paris's Bibliothèque Nationale discovered parchments known as Les Dossiers Secrets , identifying numerous members of the Priory of Sion, including Sir Issac Newton, Sandro Bottichelli, Victor Hugo and Leonardo Da Vinci."
Lynn Picket and Clive Prince comment upon Les Dossier Secrets in their book The Templar Revelation . In their opinion "Les Dossiers Secrets may appear to be complete nonsense, but the sheer scale of effort put into them and into maintaining their claims, gives one pause.... One curious feature of the dossiers is the constant and underlying implication that the authors had access to official government and police files".
Picket and Prince go on to establish that the Priory of Sion is a fake secret society deliberately set up by people with powerful connections in order to confuse those searching for the truth. At the time of writing the Wikipedia article on Priory of Sion provided a lot of good information on the subject . It should be noted that Picket and Prince go on in the second half of their book to exam the life of Jesus without any reference to solar symbology in Christianity, which means their material can not be trusted without question.
When Sophie Neveu and Dr Robert Langdon meet Leigh Teabing The Da Vinci Code introduces a number of other books, with emphasis on The Holy Blood and the Holy Grail (retitled Holy Blood, Holy Grail in the United States)  by Michael Baigent, Richard Leigh, and Henry Lincoln, which is one source of The Da Vinci Code's primary plot theme of a secret Jesus bloodline.
Michael Baigent is a Freemason and a Grand Officer of the United Grand Lodge of England, he has been editor of Freemasonry Today since April 2001, which he has used as a platform for a more liberal approach to Freemasonry. He is a trustee of the Canonbury Masonic Research Centre . Baigent's work is not therefore a reliable source of information on secret societies because he has to protect the secrets of Freemasonry.
Historian Ken Mondschein ridiculed the idea of a Jesus bloodline, writing "The idea of keeping the family tree pruned to bonsai-like proportions is also completely fallacious. Infant mortality in pre-modern times was ridiculously high, and you'd only need one childhood accident or disease in 2000 years to wipe out the bloodline; if, however, even one extra sibling per generation survived to reproduce, the numbers of descendents would increase at an exponential rate; keep the children of Christ marrying each other, on the other hand, and eventually they'd be so inbred that the sons of God would have flippers for feet." 
The real secret of the Knights Templar, which both Dan Brown and Picket and Prince studiously avoid, is that the Freemasons were created by disenfranchised Knights Templar in Scotland and England after their order had been dissolved by Pope Clement V in 1312. Thanks to the work of John J. Robinson the mainstream media are now forced to accept this fact. 
Finally it is worth noting that Dan Brown's hero's in the book are those who protect The Priory of Sion and its secret.
Angles and Demons, The Lost Symbol, Inferno
After chasing Illuminati clues across Rome in Angels and Demons, Robert Langdon finds out that the Illuminati is a fictional organisation, which is no doubt exactly what the "illuminated" one's would prefer we believe. The truth is that the original writings of the Illuminati are quite real and you can read all about them at Terry Melanson's excellent website. 
The lost symbol gives us the evil character Mal'akh who infiltrated the Freemasons in order to obtain an ancient source of power, which he believes Robert Langdon can unlock for him in return for Peter Solomon's life. All is well in the end, though as the CIA come to the rescue and it is revealed to Langdon that the Masons have (metaphorically) buried the word of God, believing that, when the time is right, its rediscovering will usher in a new era of human enlightenment. Once again secret society members are the heroes, which is a bit of a give away as to who's side Dan Brown is on. The truth is that the Freemasons intend the word of God to be buried indefinitely, which is why they do not want people to find out the truth about religion - an instrument of social control used to keep people compliant; and spirituality -a personal relationship with divine intelligence which requires no intervention by human authority figures.
Inferno tells how Robert Langdon helps the World Health Organisation (WHO) catch those responsible for a dastardly plot to create a new virus that would rewrite the human genome to cause infertility and cleanse the world of overpopulation by approximately one third. The truth is that the Rosecrucians have erected a large stone monument to their goal of reducing global population  and that in July 2013 the Japanese government withdrew support for HPV vaccines, which are promoted by the WHO  because, among other harmful effects, there is evidence that they cause infertility. 
- Wikipedia, Dan Brown, 28 April 2012, http://en.wikipedia.org/wiki/Dan_Brown
- Wikipedia, Dossiers Secrets d'Henri Lobineau, 29 April 2012, http://en.wikipedia.org/wiki/Dossiers_Secrets_d%27Henri_Lobineau
- Lynn Picket and Clive Prince , The Templar Revelation, 2007, http://www.watchmenfaithministries.com/images/The_Templar_Revelation_-_Secret_Guardians_of_the_True_Identity_of_Christ__1998_.pdf
- Wikipedia, Priory of Sion, 6 August 2013, http://en.wikipedia.org/wiki/Priory_of_Sion
- Michael Baigent, Richard Leigh, and Henry Lincoln, Holy Blood Holy Grail, 1983, http://openeye.weebly.com/uploads/2/0/6/3/2063270/holy_blood_holy_grail.pdf
- Wikipedia, Michael Baigent, 28 April 2012, http://en.wikipedia.org/wiki/Michael_Baigent
- The Sun, Home-to-the-Knights-Templar-and-birthplace-of-the-Freemasons, 29 April 2012, http://www.thesun.co.uk/sol/homepage/news/scottishnews/4279444/Home-to-the-Knights-Templar-and-birthplace-of-the-Freemasons.html
- Bavarian Illuminati, 6 August 2013, http://www.bavarian-illuminati.info/
- Rote Pille – Exsuscitati, Georgia Guidestones, 8 March 2012, http://exsuscitati.wordpress.com/2012/03/08/the-georgia-guidestones/
- World Health Organisation, Human papillomavirus and HPV vaccines, 6 August 2013, http://www.who.int/reproductivehealth/publications/cancers/IVB_07.05/en/
- The Healthy Home Economist, Gardasil Shocker: Japan Withdraws Support for HPV Vaccine, 16 June 2013, http://www.thehealthyhomeeconomist.com/gardasil-shocker-japan-withdraws-support-for-hpv-vaccine/