I wrote this years ago when the Da Vinci code first came out for a religious studies class. Surprisingly, given how much of a phenomenon the book was at the time, I was totally unaware of it until I had to read it for class. My short answer is yes he really is that inaccurate.

In the space of only five chapters of his book Brown manages to make numerous claims about Jesus, the early beliefs of the Jesus movement, Mary Magdalene, her relationship with Jesus, and the canonization of the bible. His main thesis is that Jesus’ followers believed he was a mortal prophet. One whose teachings were inspirational and who possibly may have had a claim to the throne of Israel. Brown makes several claims to support this thesis, all of which are very problematic. At the heart of Brown’s argument is the question of canonization and the political aims of canonization. Which gospels are we to trust, and not to trust? Constantine “summoned the Council of Nicaea (the First Ecumenical Council) in May 325, to settle what he termed "a fight over trifling and foolish verbal differences"” (http://www.ntcanon.org/Arianism.shtml). Brown sees this council as a politically motivated attempt to secure Imperial power by eliminating the mortal Jesus and replacing him with a divine one...

...Brown’s most important assertions to support his theme of mortal prophet transformed into divine figure involve the process of canonization. There are two basic assertions Brown makes about this process. The first is that the ecumenical Council of Nicea created the idea of a divine Jesus. According to Brown, “until that moment in history, Jesus was viewed by His followers as a mortal prophet… a great and powerful man but a man nonetheless. A mortal”(233). The second, related, claim is that Constantine had a direct hand in the canonization of scripture. Constantine’s involvement supposedly was politically motivated, an attempt to manipulate religion to enhance his own power base. It is impossible to either support or refute questions about Constantine’s ulterior motives in legitimizing Christianity. Doubtless, Constantine had at least some political motivations in legalizing Christianity, but without proper historical documentation the question cannot be properly answered. To say that until the Council of Nicea, followers of Jesus did not believe in his divinity is wrong. While there were factions within the early Jesus movement that did not believe Jesus was divine, most notably Arianism and its predecessor, the theologian Origen, this was by no means a majority position. By 200 C.E. this had already been declared a heresy. Origen was excommunicated from the his church and banished from his school in Alexandria for teaching this doctrine. The question of Christ’s divinity had nothing to do with solidification of Imperial power. Proto-orthodox thought held that since only God could redeem the earth, if Jesus Christ was not divine then his death was meaningless. Without the resurrection, and along with it its implication of Christ’s divinity, then man was still living under the burden of sin. As Paul writes, “if Christ has not been raised, then our proclamation has been in vain, and your faith has been in vain” (1 Cor 15.14). Long before the Council of Nicea, Christ’s resurrection and implicit divinity was considered by many as a necessary part of being a Christian...

...The second part of this is Brown’s assertion that the canonization of scripture was something prompted by Constantine for political reasons. In effect, Brown postulates that a political program intended to shore up the hierarchical power of the Church motivated the canonization, and the subsequent dismissal of the non-canonical gospels. With a divine Jesus, supposedly salvation could only come from the Church and consequently the emperor under whose auspices the Church operated. However, there were canonization processes already under way in the late second century C.E. We have numerous accounts of canonical lists extant from the second century. The Muratorian fragment, Irenaeus, Eusebius, and others describe in detail which gospels are worthy of inclusion and reading and which can be regarded as spurious or heretical. Many of these writings are in response to various heresies, such as the Marcionite movement, Origen’s writings, and the Valentinians. It was Origen, who was one of the predecessors to the Arianist movement, who Brown supports. Since the responses to these heretical movements came almost a hundred years after the death of Jesus, we can assume that these beliefs are a later development than the orthodox position. The vehemence of Irenaeus’s response in his Adversus Haereses, in which he specifically details and refutes the Marcionite assertions lead us to believe that these were matters of great importance to the early church, and not merely acceptable differences of opinion. Orthodox followers saw these heresies as very dangerous to the faith they held. In fact so dangerous that “when those called to martyrdom from the Church for the truth of the faith have met with any of the so-called martyrs of the Phrygian (Montanist) heresy, they have separated from them, and died without any fellowship with them, because they did not wish to give their assent to the spirit of Montanus and the women.” (Eusebius, V, xvi, 27)...

...Brown’s supposedly “impeccable research” (New York Daily News Review) is often unreliable and at best could be construed as a marketing ploy to sell an otherwise humdrum thriller. The claims he makes are not new, and the “secret” he tells is nothing more than a revisiting of the early Christian heresies of Arianism and Gnosticism. While the information that the current Christian canon was the product of political and theological infighting in the early church may be new to readers unschooled in scriptural study this fact is not a secret to anyone even slightly conversant with early church history... Brown’s research is filled with mistakes that undermines his academic credibility, even apart from those I have discussed in this paper. For example, the Nag Hammadi library, of which the Gospel of Philip was a part, was written in Coptic, an ancient form of Egyptian, rather than Aramaic as Brown states. Also Brown points to the book of Genesis as a “Christian” book, but it predates Christianity by a thousand years and was part of the Jewish tradition long before the advent of Christ and his followers. These are only a few of the assertions Brown makes that cannot even be dignified as speculation. They are just plain wrong. If Brown cannot even get simple facts straight, how reliable is his scholarship on much more complex questions, such as the early church’s canonization process, or the beliefs of the early Jesus movement?