@checkmate--What you said to TheLastCenturion. (Hi Rory!)
No, Constantine was not the first important Christian convert. However, he did in fact pretty single-handedly make Christianity into one of the major religions of the Mediterranean. Before his successful bid for the purple: illegal. By the time he died: Powerful. Two emperors later, Julian 'The Apostate' actively strove to revive the old religion, and failed due to lack of public interest. Christianity was an incredibly rabid meme, and once Constantine gave it open ground, grew like kudzu.
Constantinople, the city he created on the site of old Byzantium, had state-funded churches. His mother Helena went to Israel and brought back what is traditionally understood to have been the True Cross.
The history is unclear on many points, like what he personally believed, but that he was the great champion of the religion there can be no doubt.
The man designed the basilica in Constantinople with thirteen pillars, twelve of them containing relics of the faithful apostles and the last being designated as his own tomb. He was basically talking up Christianity as a replacement cult-center for the Empire, given the one they'd been using didn't seem to be inspiring much passion anymore, and Rome was incidentally falling apart, which was why he was fighting another claimant with actual armies in the first place.
And you know what? It worked. Sure, the West declined into semi-barbarism, for the same incredibly complicated systemic reasons it'd already been declining and a series of weak emperors. But the Church became a cornerstone of the Eastern Empire for another thousand years, over five hundred of them powerful and prosperous, if never quite the juggernaut of the golden age, since they had the Persian empires to deal with.
All that said, charlsby's history also sucks, for reasons I've partially enumerated, and he's a moron. But all the people falsely linking the fall of the empire to its conversion, here on this site: that is wrong. That is sloppily researched correlation used to assume causation, and it's every bit as lame as claiming the Romans fell because they were morally dissolute and had orgies, and undermines the argument that atheists are more rational than theists, and would you all please stop?
Bad historiography makes me very sad. Heh, we all have something that's sacred to us, I guess.