The internet is full of errors, misinformation, and quackery. Also, people are frequently willing to believe what they are told is true without bothering to check sources or see for themselves if the information being presented has any veracity. Also at work here is good old fashioned confirmation bias.
None of these things are unique when it comes to misinformation and quackery as it pertains to the history of Christianity, but it is present there as well. Which is why people believe hacks like Alexander Hislop, Jack Chick, and Dorothy Murdock (aka
Acharya S) in spite of the fact that these folks provide zero evidence to back up their claims, because there is no evidence to back up their claims.
Such misinformation can range the from the straight up absurdist (Constantine literally invented Christianity) to the more subtle but equally as fallacious (Constantine allowed and/or forced pagan elements to enter into Christian practice).
When it comes to, for example, various claims that there were a lot of virgin-born savior gods who were crucified and rose from the dead it's easy to point out the falsehood in these by simply going back and looking at what ancient people actually believed about such figures in their own mythologies and stories. Mithra emerged from solid rock, Dionysus was the product of sexual relations between Zeus and a human mother, Siddartha (Buddha) was the natural offspring of his parents, Krishna was the eighth child born to his mother and (very human) father. Osiris wasn't crucified and didn't rise from the dead, but instead was reborn as lord of the underworld; Horus wasn't born of a virgin but was the result of sexual union between Isis and the reassembled and magically enhanced corpse of Osiris. And so on and so forth.
The earliest mention of the Nativity feast of Solis Invictus is in the mid 4th century, and there's no evidence that Christians borrowed it to celebrate Christmas, and instead Christians were debating Christ's birth well before the beginning of the Solis Invictus cult. Easter has nothing to do with pagan fertility goddesses, the word "Easter" gets its name from the name of the month, Eostermonath; the name of the month (according to the Ven. Bede) comes from a supposed goddess named Eostre, but Bede is literally our only source and most of the names of the old Anglo-Saxon months are based on natural phenomenon, not gods--besides the majority of Christians have never called Easter "Easter" but use the more ancient term "Pascha" or variant thereof. "Easter" is a uniquely English term shared only with German "Ostern", even in other Germanic languages a variation of "Pascha" is used, such as the Dutch "Pasen", the Icelandic "Paska", the Faroese "Paskir", the Swedish "Pask", and so on.
And it really can go on and on. Some of the problem is that certain bits of misinformation just keep getting repeated over and over and becomes part of the cultural "common knowledge" landscape, sort of like lots of people still think Columbus set sail to prove the earth was round even though everyone in Europe already knew that, we've known the earth is round since the time of the ancient Greeks, hundreds of years before Christ. But these things just self-perpetuate. Which is why we get "Of course Constantine started [catholic] Christianity" or "Of course Christmas, Easter, and Halloween are pagan" and so on. Think of how often people still think the abbreviation "Xmas" is about "taking Christ out of Christmas" even though the "X" in Xmas is a monogram for Christ, it is the Greek letter Chi, not an "X", it stands for "Christ" and Xmas is a very old abbreviation for Christmas. Xmas is not "exmas" it is "Christ-mas".
-CryptoLutheran