The most objectional claim of The Da Vinci Code is not that Jesus had a child with Mary Magdalene. Certainly, there is no historical evidence that Jesus and Mary had a child, and consequently I reject the claim on that basis. Still, if Mary and Jesus had a baby, I don't know if that would necessarily have any major impact on my faith or the faith of thousands of other Christians. After all, the central claim of Christianity is not that Jesus Christ was celibate, but that Jesus Christ, God-incarnate, died on the cross and rose again from the dead to save everyone who believes in Him from their sins. But, of course, that is exactly the claim that The Da Vinci Code attacks.
Slate Magazine has a nice article responding to this claim entitled "Ungodly Errors: Scholarly gripes about The Da Vinci Code's Jesus" by Larry Hurtado, Professor of New Testament Language, Literature & Theology at the University of Edinburgh. Dr. Hurtado notes:
The belief that Jesus is somehow divine was not invented by the Roman Emperor Constantine in the fourth century, as Brown and movie director Ron Howard have Teabing say. Instead, this belief is attested in first-century Christian texts, such as the Gospel of John, and dates back even earlier to the letters of the apostle Paul, whose New Testament writings between A.D. 50 and 60 are the earliest Christian texts we have. Faith in the divine glory of the resurrected Jesus appears to have emerged amazingly soon after his execution, most likely among circles of his Jewish followers. Scholars commonly regard particular passages in Paul's letters as preserving early hymns about Jesus, in which he is praised as the one through whom the world was created, and as sharing in God's nature and glory.
In fact, in pretty much the entire body of early Christian writings from the first three centuries, Jesus' divinity is taken for granted. Christians differed not over that basic assumption but rather over how to understand his divine nature. At the Council of Nicaea in A.D. 325, the crucial question was how to reconcile Jesus' divinity with Christian monotheism.
Curiously, The Da Vinci Code presents the so-called Gnostics, who regarded other Christians as lesser beings than they and were in turn treated as heretics, as the heroic defenders of a thoroughly human Jesus. But actually the historic Gnostics and the gospels often linked with their circles did not emphasize Jesus' human nature at all—quite the opposite. Typically, Gnostic Christians portrayed Christ as a heavenly being who came down to earth to awaken them from their spiritual slumber by disclosing their own divine inner nature. Regarding the physical world as a source of delusion and place of confinement, Gnostics were deeply negative about bodily existence, including their own. So, they tended to treat Jesus' body as simply the temporary vehicle for his revelatory mission, believing that he discarded it before returning to his heavenly status in the realm of pure light. It was actually the Orthodox Christians who made much of Jesus' full human nature and the reality of his death as the essential redemptive act.
The article continues to point out the error made by Dan Brown in assuming that the New Testament canon was derived at the Council of Nicea. Dr. Hurtado writes:
To clear up another piece of history on which The Da Vinci Code is completely unreliable, the New Testament was not created at the Council of Nicaea in A.D. 325. The question wasn't even on the council's agenda. The formation of the New Testament had begun much earlier and continued on later than Nicaea. The familiar four Gospels, which scholars commonly regard as the earliest such texts, were treated as a completed set at least by A.D. 150 in many or likely most Christian circles. Still earlier, Paul's letters were collected and circulated as scripture. In the early third century, the Christian scholar Origen listed the writings regarded by most Christians of his time as scripture, other writings that had largely been rejected, and others still under consideration. Among the texts regarded as scriptural, he included most of those that became part of the New Testament.
It's also important to emphasize that this question of which writings to treat as scripture, which to treat merely as edifying reading, and which to regard as heretical, was not decided at a single point by a church council, a pope, or a Roman emperor. Once again, in service of its conspiracy theory, The Da Vinci Code gets it wrong. The canonizing of scripture involved circles of believers spread across the many lands of the Roman Empire and beyond. The result wasn't a fiat foisted upon the Christian world. Essentially, the writings that commended themselves earliest and to the largest number of Christians came more quickly and securely to be part of the emergent New Testament.
The idea that the Council of Nicea either made Jesus divine or somehow selected the canon of the New Testament out of thin air at the behest of Emporer Constantine is simply wrong.