Post-Modern geniuses usually attack an opponent, oftentimes quite spuriously, as a 'flat earther'. This is a rather curious ad-hominem considering that the Flat Earth Society's President believes in the fiction of evolution and globaloneywarming. Behind the slander – wrongly directed usually - is the concept that medieval Christians were 'flat earthers', licking the dirt, moaning to their idols, and completely ignorant of the world around them – until of course that god of science Galileo strutted upon the scene. Then suddenly, magically, the world of thought, inquiry and science was transformed......
Factually no one believed in a flat earth after circa 300 B.C. including Christians, who discovered modern science, including calculating and defending long before Columbus, the circumference and sphericity of the Earth.
The Ptloemaic universe, created by the Greeks in Alexandria in the 1rst century AD had accurate calculations of the earth's circumference. Alexandria was by the 3rd century a Christian-Greek city, wiped out by the Moslems circa 642 AD. Ptolemy was not a Christian but many of his associates, students and acolytes were certainly Christian. The observational data around sphericity and even heliocentricity long predate Galileo and the 'Enlightenment', which was in many ways, quite unenlightened.
Isiah in the Old Testament for example mentions the spherical nature of the earth [he that sitteth upon the sphere...]. Christianity has never been opposed to natural reality. There is no proof that Christians in any era en-masse, believed that the earth was a piece of flat-bread, floating in the ether [as per Aristotle who wrongly believed in geo-statis, and cosmological fixity]. Christian philosophers and scientists who discussed the 'roundness' of the globe in their writings, letters and books include [see also Garwood's Flat Earth: History of an Infamous idea, here]:
-Boethius 5th century
-Bede 7th century
-Alcuin 8th century
-Peter Lombardus 12th century
-Duns Scotus 13th century
-Christian Scholastics – various - from 13th to 15th centuries [See Grosseteste 13th c.]
-Thomas Aquinas 13th century
-Dante 13th century
-Pierre D'Ailly 14th century
[used by Columbus to prove that the voyage could be profitable, see the book 'Columbus the Intrepid Mariner, by Sean Dolan here]
Only 2 minor Christian writers rejected the early Greek, Christian discoveries on the earth's shape. One was the ineradicably dumb Lactantius (AD 245-325), a professional rhetorician who converted to Christianity in his mid-life for reasons unknown and who rejected current Church teaching that the earth was a globe. His minor writings were dug up during the Renaissance and used as a cudgel against the Church. He had no following in his own life. A second ignoramus was the 6th century Greek Christian 'Cosmos', who likewise rejected reality and proof and opined about a flat earth as being conversant with the Bible [it most certainly is not]. Both of these men were declared heretics by the Church and their works forgotten until the early modern period.
For the record, Moslems will offer that a converted non-Arab Al Bruni 'found' the circumference of the earth. This is nonsense. Greeks and Christian astronomy and mathematical inferences pre-date Bruni by 700 years. Moslems offer nothing in science and technology to compete with the 200 Christian inventors and scientists one can name from 500-1500 AD. The idea that Moslems 'calculated' the earth's sphericity is a lie. Christians and non-Moslems had translated extant Greek sources of Aristarchus and Eratosthenes into Arabic but Moslem astrology which never mutated into astronomy, is remarkably meagre. In any event, ascribing to a Moslem, a known-world view which pre-dates Islam by 700 years is remarkably dumb.