Common Errors (40): Constantine’s Conversion � New at LacusCurtius & Livius
"At the end of his life, Constantine was indeed a Christian. If we cannot accept Eusebius’ statement that the emperor was baptized several days before his death in 337 – and some methodological scepticism is always prudent – we can deduce the emperor’s beliefs from the tomb he had designed: he was to be buried in Constantinople, together with relics of the twelve apostles. In other words, Constantine wanted to be commemorated as equal of the apostles (isapostolos) and as a second Christ – perhaps a bit blasphemous for modern Christian sentiments, but not below the standard of a Roman emperor, who was a dominus et deus."
The evidence, many centuries on, is a mass of confusion and propaganda, but I have always understood that it became normal at the time to convert to Christianity only on the death-bed so that one could more easily agree not to repeat past sins that were necessary as part of the job of being emperor or general. Another theory is that he had intended to baptised in the River Jordan, like Jesus, but this plan fell through (see here and here).
I also believe that there was confusion, perhaps deliberate, between Sol Invictus and Christianity. There is an obscure literary source which could back up the theory of Constantine having mistakenly thought that by embracing Christianity he could be the living Christ but of course I can't think where it is ... His tomb would be an example of archaeology supporting such a theory. Whether it's true or not, who knows. And frankly I'd rather not venture off into Da Vinci Code territory ...