Step 1: Have a barely literate, preferably drunken teenager recount to you his half-remembered reading of Umberto Eco's Foucault's Pendulum, The Name of the Rose or Baudolino—all great novels dealing with esoteric distinctions in theological history, battles that these caused, conspiracy theories, secret societies both real and imagined, etc., and all undergirded by research—while paying as little attention as possible. This will replace you having to actually think up a plot or do research of your own.
Step 2: Read some New Age reinterpretations of either medieval mystics or "Eastern thought". While you are at it, learn everything you can about science from blogs on the Web—this might help you to think that 17th century scientists believed in the four elements. Come to think of it, you can do your research about religion on the Web, too.
Step 3: Forget everything you have ever known about the way that actual people act or talk. It is essential that you avoid all real human motivation.
Step 4: Invent a ridiculous academic discipline. Brown's choice is "Symbology", but you can pick your own. Just make sure that this discipline has an honored chair at Harvard, Princeton, Oxbridge, somewhere famous. Also, make sure that people respect the professors of this discipline; this is called "suspension of disbelief", since professors of disciplines are not respected. Finally, make sure that the deep wisdom this discipline makes available is of the sort to deliver common-sense wisdom and obvious pieces of information, while everyone else is totally unaware of what is nearly smacking them in the face. This helps the reader/viewer feel intelligent.
Step 5: Take some Ambien, don't let yourself fall asleep and begin writing.
Step 6: Wait for Tom Hanks and Ron Howard to call.