Think back to your school days for a moment. Do you remember when you first learned about Christopher Columbus? If so, do you remember what you were taught? If you're like a lot of people, you heard that those of Columbus' day, particularly Christians, believed that the world was flat and that if Columbus sailed far enough he would fall off the edge of the earth. As Bruce Walker points out in an article for American Thinker, the story feeds the narrative that Christianity and science are inherently antagonistic to one another, since Christianity hinges on faith and science supposedly hinges on reason. It is a narrative that is still actively promulgated today among proponents of evolutionary theory.
Problem is, that supposed antagonism, as well as the idea that Renaissance and Medieval-era Christians believed in a flat earth, is a myth. An objective study of evolutionary theory reveals that it relies much less on empirical science than on extraordinary leaps of faith. And unlike creationism, it has no explanation for how the universe came into being.
Likewise, a study of the historical record reveals that the idea that people of the Middle Ages thought the earth was flat dates only to the 19th century. Before that time the truth was widely known: that educated people, including Christians, have for 2000 years known that the earth is round.
So what happened in the 19th century that started the myth of Christians believing in a flat earth? And why has it had such staying power?
You can read more about the first question here. As for the second question, I think part of it is just the storybook element. Human beings are suckers for irony and surprise endings. So the thought of an intrepid sailor heading off into the unknown while a world watches and waits in fear for him to fall off the edge of the horizon is an image we can't resist. The word "history" has "story" in it, and the best history, the kind that stays with us, is made up of larger-than-life stories. But when those stories are mythical rather than factual in nature, we end up with a false historical record. Because once a story is introduced and embraced, it takes on a life of its own, and the truth or lack thereof matters little.
But in this case there is something more going on than a riveting story. In this case the story was an intentional deception by someone with an agenda. Others with the same agenda latched on to it and are still latching on to it today. The stereotype of Christians as vacant-eyed, stupid, unthinking automatons is still with us, perhaps now more than ever before. Never mind that one of the foundations of Christianity is a belief in an orderly and understandable universe rather than one that is random and filled with mutations and "mistakes." If you ask me, the former sounds a lot friendlier to science than the latter.