There's the question of the historical Jesus, enthusiastically discussed by more and ever more laymen, and left undiscussed by Biblical scholars and Christian theologians scholars who still almost unanimously insist that the matter has been thoroughly investigated (When? Where?) and that it's certain Jesus existed, and Shut up!
Then there's the entire excitement surrounding Constantine the Great, the inaccuracies about him which are so popular: It's still so often said that he made Christianity Rome's official religion -- he did not. It's said that he (often: he and the Pope) wrote or re-wrote or edited the New Testament at the Council of Nicea. Nope: the Pope wasn't there; the Pope and Constantine had many more reasons to be enemies than to be allies; nobody altered the Bible or discussed what should or shouldn't be in it at Nicea; and there's no evidence that Constantine gave a rat's ass one way or the other about what was in it.
Here's a question which might deserve much more study than it has generally received so far: would Constantine have involved himself with Christianity at all if his mother, the empress Helena, had not been a Christian? I put it to you: which seems more plausible: that a Roman Emperor who, all who have studied his life agree, was a particularly savvy politician, that this Emperor gave some support to Christianity because, at a crucial battle in his struggle to solidify his control of the Empire, he saw a cross in the sky along with words telling him that with this sign he would conquer -- or that he gave some support to Christianity because his mother was a Christian and had a lot of influence on him?
The story of the cross and the words in the sky, and a lot of other nonsense, comes from Eusebius, who unfortunately is our most important single surviving source of the history of Christianity up until Constantine in general, and of biographical information about Constantine in particular. I say unfortunately because Eusebius' pants were on fire. I say unfortunately because the truth was not in him.
Some apologists and conservative historians will attack me for doubting the veracity of Eusebius, but that's okay. I'm in very, very good company: Edward Gibbon's multi-volume History of the Decline and Fall of the Roman Empire
has been praised as a groundbreaking work of genius, still unsurpassed in many ways two and a half centuries after its first publication -- because that's exactly what it is. It has also been vehemently condemned from the time it first appeared uo until the present day -- because Gibbon was clearly (although not quite explicitly) an atheist, and because he dared to question the accuracy of the historical accounts given by people like Eusebius.
A century after Gibbon, Jacob Burckhardt, another historian of great genius, enjoying the greater freedom of expression given to us all by courageous pioneers of freethinking like Hobbes and Spinoza and Hume and Gibbon and Voltaire, found no reason to hide his great annoyance with Eusebius, who had so thoroughly hidden and blurred the history which he, Burckhardt, was working so hard to find. Burckhardt came right out and called Eusebius a liar. and of course, the same people who disliked Gibbon also attacked Burckhardt, for the same reasons.
But lo and behold great wonders, O ye nations: as time passes, Gibbon and Burckhardt look more and more reasonable, as Eusebius, whose veracity was even attacked by other Christian historians as early as the 5th century, looks more and more like a teller of tall tales and less and less like the historian he called himself, and for which he was mostly taken from his time to Gibbon's.
And this man, Eusebius, is pretty much the founder of Christian historicism, the foundation upon which much of the history written over the course of the next millenium in Christendom, was based. Gibbon and Burckhardt and anyone else who cared about investigating history properly were quite right to be annoyed. Such a shaky foundation has produced a lot of spectacularly shaky results, and continues to do so today, although, as I said, Eusebius' falsehoods are finally beginning to be exposed and undone.
So I would say, to those who dislike Christianity and its continued omnipresence and power: don't blame Constantine above all others. If it hadn't been for his mother, he might never have given any support to Christianity. He might have continued Diocletian's persecution of it, and you and I might never have heard of Christianity. But far more, blame Eusebius, who took Constantine's support of Christianity and said that it was a conversion to Christianity, although Constantine never withdrew his support for the pagan religions. Blame Eusebius for intensifying the Christian disregard for reality and reason. Blame Eusebius for spreading the idea that Christianity had conquered Rome, decades before it actually did. Reality and reason and historical accuracy were defeated first, and then the Empire followed.