The Holy Grail
To properly consider the answer the place to begin is the question of the Holy Grail itself. Brown suggests that the Grail legend is a metaphor for the womb of Mary Magdalene. As evidence he suggests that the French San Greal, Holy Grail, was a mistake and that it should actually be Sang Real, Royal Blood. As enticing as this idea may be, it's wrong. To understand the problem of this it is important to realize several things. First, the significance of the grail cannot be found in the bible. While Jesus did bless a cup of wine transfiguring it into blood, no mention of this cup appears again. Further, no reference exists to Joseph of Aramathea, or any one else, catching Christ’s blood at the crucifixion. Even more significantly, the word originally used was San Graal. The word Graal comes from the Latin gradalis meaning a dish brought to the table during various courses of a meal. In its oldest context, the Grail was not a cup (defeating any efforts to suggest a symbolic connection to the female womb). This is further supported by the actual text of Cretien De Troyes "Perceval, the Story of the Grail", the oldest documented Grail story. In this story he describes the San Graal as a golden dish carrying a communion wafer.
Their some even larger problems with this idea. First, is that Chretien De Troyes Parcivel is the first known mention of the Grail. It was written some time between 1180 and 1191 AD. So if a blood line existed its first mention in any text is over a thousand years after Christ died. Even more damning Perceval and other early Grail stories do not link the object with Jesus Christ in any way. Robert De Boron writing a the end of the 12th century is where the link between Christ and the Grail actually began.
Was Christ Married?
Yet the question remains what of the idea he married Mary Magdalene. Any one who has read the New Testament knows that this idea is not mentioned at any time. Brown's answer is to tell us that the Catholic Church and Emperor Constantine destroyed all references to it and that the Gnostic texts (heretical works most scholars believe were written between 110 and 300 years after the death of Christ). The closest reference made is in the Gospel of Phillip that says that Jesus and Mary Magdalene were companions. One could still argue that the marriage did occur but with a complete lack of any historical evidence its pure speculation not a valid theory.
Another argument is that Jesus had to be married because he was a Jewish rabbi but this is also wrong. First Jesus was not a formally trained Rabbi. He was called rabbi by his followers out of respect but that’s not the same as the formal position. Also some Judaic sects, notably the Essenes, practiced celibacy during Christ's time so it’s clear it would not have been absolutely necessary for him to marry.
The larger question about this is why would the Roman Catholic Church have suppressed this information?? Nothing about a lawful union is a violation of the laws of either the Church or the Old Testament. The answer usually given is that they were trying to weaken the importance of Mary Magdalene in particular and women in general. If this were the case then the Gnostic Gospel of Thomas would have been an ideal addition to the bible. It contains a quote attributed to the apostle Peter saying: "Let Mary go away from us, because women are not worthy of life." Jesus even goes so far as to say he will make Mary a man so she can enter the kingdom of Heaven. The New Testament is far kinder on women than it is generally given credit for. It was women, Mary Magdalene among them, who first saw Christ resurrected. Also notably it was Jesus's mother Mary was among the very few people with the courage to stand by him through the entire course of the crucifixion. There are also references in the New Testament that further dispel the idea they are anti female. Consider Galatians 3:28 that tells us that there is no longer man or women but all are Christians.
The Council of Nicea
The greatest claim perhaps is the idea that Jesus was not in fact divine but simply a prophet. The two big issues here relate to the council of Nicea. Called by Emperor Constantine this was a meeting of over three hundred bishops to discuss a number of important issues. One of the major questions raised was the nature of Christ. While Christians had worshiped him as God since the 1st century, the Eastern Church Leader Arius said that Jesus was a separate being from God. The council after considerable debate voted on the matter. Of the 318 bishops gathered, only two ultimately voted against the decision that Jesus and God the father were one. This should make it clear that the majority of Christians in the early world accepted that Jesus was God. It's extremely important to also realize that even Airus considered Jesus divine. He argued that Jesus was a lesser divinity to the Father not a mere human.
Another important question related to the council was the formation of the New Testament. It’s often been questioned why the 27 books were chosen. Were these politically expedient for the church or were they accepted as the most accurate? Regardless of those involved in the council who selected them early church fathers also considered these books accurate. Even more surprisingly, heretics accepted them. For example 200 years before the council the heretic Marcion considered 11 of the 27 total books to be accurate. 140 years before the council, Irenaeus wrote regarding them and dismissed them as heretical. Even closer to the death of Christ we have Church Father Jerome who in 95 verified the four gospels of the bible along with the other books as being valid accounts. This is especially significant since it means that the New Testament as we know it would have been already known and established by then which supports the idea that it was written by eyewitnesses.
On the other hand the Gnostic religion, which created the Gnostic gospels,, The Gospel of Thomas, The Gospel of Mary, the Gospel of Phillip, and so on. The Gnostic religion was a combination of eastern mysticism and the occult that adopted some aspects of Christianity into its own beliefs. The Gnostic texts unlike the New Testament are all date from at least 110 AD (none were mentioned by Jerome or any one earlier then this date). Long after the life and death of Christ.
The Templars and the Priory of Sion
No discussion of Brown's work would be complete with out talking about the Priori and the Knights of Templar. These two groups, one from the middle ages and the other from the 20th century are at the heart of many popular conspiracy theories.
The older of the two the Knights Templar were an order of Christian knights founded to protect pilgrims on the way to the holy land. Formally recognized by the Church in 1129. From that point it spread through much of Europe becoming extraordinarily rich and powerful. In 1307 the order was accused of a long list of crimes and most of them were burnt at the stake.
Conspiracy theorists tend to argue that the knights uncovered some secret information that the church wanted to keep secret, i.e. the blood line of Christ, but their is a much simpler explanation in the form of King Phillip IV of France. Phillip, who was the driving force behind the arrests of the Templar’s, He pressured Pope Clement the V to take action against the order not because of some heresy but out of the very human emotion of greed. The Templar’s were extremely rich and when they were destroyed Phillip was able to take a large ammount of their treasury for himself.
The Priory of Sion on the other hand is a modern organization, founded in 1956, that claims ancient roots. This organization deposited a document called "Secret Dossiers of Henri Lobineau" and several other allegedly medieval texts in the Bibliothèque Nationale de France. It is from these documents that Brown's claim that Leonardo Da Vinci was a member of the Priory. The problem is these documents have long since been proven to be forgeries and no link between Da Vinci to the Priory exists, particularly since Da Vinci died before the Priory was invented.
This is just a short list of the numerous lies and misstatements Dan Brown makes to promote his book and ideas. Many more exist that are less relevant to the central point. Some additional examples: Isis was married to Osiris not Amun, John is the figure seated next to Jesus in the painting "The Last Supper" (the extremely feminine depiction of John is common in paintings from the Renaissance), and the Mona Lisa was more likely a depiction of Lisa del Giocondo (a family freind of Da Vinci). In short the book is a work of fiction from beginning to end and any resemblance between it and historical events is purely in the mind of the author.