In order to answer this question we need to first examine the motivation for such a move. As stated in The Da Vinci Code the circumstances leading to Constantine's decision to unite the empire under a modified Christian religion was the 'growing religious turmoil' between pagans and Christians who had begun warring with each other. Constantine, the consummate businessman that he was, simply picked the 'winning horse' (55.313).
Several aspects of this historical description are inaccurate. First, it is difficult to speak of paganism as a unified system of belief. Pagans came in all shapes and sizes and probably would have fought amongst themselves over differing beliefs!
Second, though there was conflict between pagans and Christians it was not, strictly speaking, a war. Instead it was a situation in which Christians were being persecuted by the pagan emperors.
As Teabing asserts, Christians had multiplied since the days of Jesus (though not exponentially as the religious historian suggests) but still only constituted a minority in the empire.
They had attracted attention to themselves through their beliefs and practices. Their refusal to attend the circus or theatre, enlist in the army, or worship the gods and goddesses who were responsible for the prosperity and well-being of the empire was considered treasonous. Rumours of cannibalism and incest, based on misunderstandings of the Lord's Supper, circulated about them.
Early persecutions tended to be sporadic and localised until the middle of the third century. The most universal and systematic of these occurred just prior to Constantine's ascension and continued unabated for eight years. Many Christians were martyred for their beliefs and others were tortured and imprisoned in an attempt to convince them to recant and burn incense to the emperor.
The persecutions ended, not because Constantine decided to unify the empire under one religion, but because Galerius, the instigator of the persecution, suffering from a terrible disease, issued an edict of toleration in 311.
Constantine and his co-emperor Licinius would issue another edict in 313. This points to the third inaccuracy contained in Teabing's outline in which he suggests that the conflict continued until 325.
What should be clear is that the motivation for creating a new religion, if this is what happened, is not quite what Dan Brown would like us to believe. It was not a war between two great religions which threatened to split the empire. There was, in fact, no compelling and practical reason for Constantine to favour Christianity at all.
Christians tended to come from the lower levels of society and didn't boast many rich and powerful people who support Constantine may have desired.
A careful reading of what Langdon and Teabing have to say about what Constantine did reveals a further problem with the theory of religious fusion; they can't seem to agree on what happened.
In Chapter Six Langdon tells Bezu Fache that Christianity waged a successful war against paganism in which the Roman Catholic Church sought to eradicate pagan symbols (6.62). In Chapter Fifty-Five, however, Teabing informs Sophie of a very different process.
Constantine converted the sun-worshipping pagans to Christianity. By fusing pagan symbols, dates and rituals into the growing Chrsitian tradition, he created a kind of hybrid religion that was acceptable to both parties. (55.314)
Two paragraphs later, however, Teabing declares that, 'Nothing in Christianity is original'. Everything from the Virgin Mary to the date of Christmas to halos are vestiges of paganism. There are three humungous issues with this reconstruction.
First, what does Dan Brown believe happened? Did Christian symbols vanquish their pagan counterparts? Did Constantine fuse some Christian symbols together with some pagan symbols? Or did pagan symbols 'transmogrify' into the Christian symbols we know today? It seems to me that these are mutually exclusive options. If Christianity was the 'winning horse' and it was fused into a hybrid religion shouldn't there be something original in Christianity?
Second, the assumption that all Christian symbols have pagan antecedents is the perfect illustration of what I have called Langdon's Hypocritical Theory of Dynamic Symbolism. Very early in the novel Langdon is asked what a certain symbol means. He replies by saying that, 'Symbols carry different meanings in different settings.' (66) In other words, the context in which a symbol is found must be taken into account to determine its meaning.
What makes his theory so hypocritical is that he consistently ignores the Christian context of many symbols instead interpreting them as evidence of barely covered paganism! Furthermore, as Olson and Miesel point out, similarities between symbols does not prove dependence and if it does there is no reason why Christianity could not have influenced paganism.
Third, and this is the biggie, what sort of religion, especially the 'winning horse', would be happy with the sort of compromise suggested by Brown. Christians had only recently been persecuted for their exclusiveness and refusal to participate in many aspects of pagan culture.
Would those men and women who had lost loved ones or who had themselves been imprisoned and tortured accepted a hybrid in which nothing of their religion was carried over? What is suggested is so highly improbable that it ought to be rejected immediately.
Did Constantine create a hybrid religion? No. Absolutely not.