Is Dan Brown's claim that the descriptions of the artwork, architecture, documents, and secret rituals depicted in this novel are accurate, true? Well, he has backed off a bit on his website to say that they merely exist, but the book can't be changed at this point. The problem is that he failed to maintain objectivity about his historical claims, and let his fictional characters follow a biased interpretation of history. This commits the error he claims the church has made throughout history. This error is referred to as relativistic hypotheses. It suffers from self-contradiction, logical fallacy, and inconsistency.
Personal bias distorts the interpretation of facts. So how can we avoid that mistake? You must find historical evidence, which is usually testimonies of eyewitnesses, written documents as close to event as possible, and physical evidence from archaeological finds. Assemble the evidence, and examine the possible alternative conclusions, setting your own personal beliefs aside for the moment. There is primary evidence and secondary evidence. Primary holds greater weight. Always check authenticity, and use external criticism to check it. Also use internal criticism to analyze the docment's reliability, or to use the scientific dating methods for verification. In the end, all the pieces of the puzzle need to be assembled, and a conclusion made based on the most likely conclusion.
Many historians that have tested biblical history, following this standard, have produced results that support the Bible's account of history. Those conclusions have never been proven wrong, not even once. Can Dan Brown's claims stand up to that scrutiny?