Veritas.ucsb.org debunks the secular lie that the Medieval period was a dark age in which Christians believed the earth was flat:

A curious example of this mistreatment of the past for the purpose of slandering Christians is a widespread historical error, an error that the Historical Society of Britain some years back listed as number one in its short compendium of the ten most common historical illusions. It is the notion that people used to believe that the earth was flat–especially medieval Christians.

It must first be reiterated that with extraordinary few exceptions no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat…….

A few–at least two and at most five–early Christian fathers denied the sphericity of earth by mistakenly taking passages such as Ps. 104:2-3 as geographical rather than metaphorical statements. On the other side tens of thousands of Christian theologians, poets, artists, and scientists took the spherical view throughout the early, medieval, and modern church. The point is that no educated person believed otherwise.Historians of science have been proving this point for at least 70 years (most recently Edward Grant, David Lindberg, Daniel Woodward, and Robert S. Westman), without making notable headway against the error. Schoolchildren in the US, Europe, and Japan are for the most part being taught the same old nonsense. How and why did this nonsense emerge?…………

No one before the 1830s believed that medieval people thought that the earth was flat.

The idea was established, almost contemporaneously, by a Frenchman and an American, between whom I have not been able to establish a connection, though they were both in Paris at the same time. One was Antoine-Jean Letronne (1787-1848), an academic of strong antireligious prejudices who had studied both geography and patristics and who cleverly drew upon both to misrepresent the church fathers and their medieval successors as believing in a flat earth, in his On the Cosmographical Ideas of the Church Fathers (1834). The American was no other than our beloved storyteller Washington Irving (1783-1859), who loved to write historical fiction under the guise of history………..

But now, why did the false accounts of Letronne and Irving become melded and then, as early as the 1860s, begin to be served up in schools and in schoolbooks as the solemn truth?

The answer is that the falsehood about the spherical earth became a colorful and unforgettable part of a larger falsehood: the falsehood of the eternal war between science (good) and religion (bad) throughout Western history. This vast web of falsehood was invented and propagated by the influential historian John Draper (1811-1882) and many prestigious followers, such as Andrew Dickson White (1832-1918), the president of Cornell University, who made sure that the false account was perpetrated in texts, encyclopedias, and even allegedly serious scholarship, down to the present day. A lively current version of the lie can be found in Daniel Boorstin’s The Discoverers, found in any bookshop or library.”

And why did these historians spread propaganda against Christianity?

It was in order to shore up the arguments of Darwinists  (with whom Darwin himself would have disagreed) against Christians.

It was to make Christians look stupid and opposed to science.

It was to falsely equate a non-existent “Christian” belief in a flat earth with anti- Darwinism so as to shame educated Christians into subscribing to the dogma of evolution, without fully grasping the import of what they were doing.

It was subversion of religion by stealth.