Brown certainly has an agenda against the Catholic Church. Was he expelled from the Catholic church? Or is he Muslim?
He does echo Muslim slanders against christianity, but Brown's ideology is borrowed chiefly from a French Nazi and a British witch.
http://bestsellers.about.com/od/authorprofilesaz/p/dan_brown.htm
That's a sketchy bio but if his background is Phillips Exeter and Amherst, Catholic or Muslim seems unlikely.
More likely a mainline Protestant if he ever associated with religion.