The Da Vinci Code claims that Constantine's conversion was not authentic but that he remained a life long pagan who continued to worship the Unconquered Sun and who was baptised against his will on his death bed. Bart Ehrman, head of the Department of Religious Studies at the University of North Carolina, states that though Teabing is correct to intimate that the emperor was not a 'tried-and-true Christian with nothing pagan left about him' it is also clear that he 'began to see himself as a Christian in some sense' (2004, 10-11). Christian historian Justo Gonzalez agrees and describes Constantine's conversion as a 'long process' that began in the year 312 at the battle of the Milvian Bridge on the outskirts of Rome (1984, 107).
On the night before the battle Constantine had a vision to place a Christian symbol on the shields of his soldiers. The symbol was the labarum which is the conflation of the Greek letters chi (C) and rho (R) - the first two letters of Christ's name. The next day Constantine defeated his enemy and took conrol of Rome. The next year he met with the emperor of the east, Licinius and issued what is known as the Edict of Milan. One of the stipulations was that persecution of Christians cease and their property be returned to them. As you can imagine this was hailed as a God-send by the persecuted Christians. Incidentally, the edict of toleration was for Christians, Jews and pagans. Christianity did not become the official religion of the empire - to the exclusion of other religions - until nearly fifty years later
Constantine's Christianity seemed to be fairly pragmatic. In Gonzalez's words,
The truth is probably that Constantine was a sincere believer in the power of Christ… when Constantine enacted laws in favor of Christianity, and when he had churches built, what he sought was not the goodwill of Christians, but rather the goodwill of their God. (1984, 122)
His ongoing association with paganism was due in part to his role as emperor. He would not have been able to suppress paganism without incredible opposition.
As Teabing asserts the emperor was baptised on his death bed but certainly not against his will. This reflected common practise at the time in which people put off the cleansing effect of baptism until death. What is important for us to note is that his contribution to Christian history was not doctrinal but political. He changed the legal status of Christianity but didn't change what Christians believed or taught. Therefore, at the end of the day the genuineness of Constantine's conversion is a matter between himself and God.