Psalter map, London, British Library, Add. MS 28681, fol. 9, London? After 1262
Apparently the misconception became popular knowledge after in 1828 Washington Irving's highly romanticised biography, A History of the Life and Voyages of Christopher Columbus, was published and mistaken by many for a scholarly work. In Book III, Chapter II Irving gave a largely fictional account of the meetings of a commission established by the Spanish sovereigns to examine Columbus's proposals. Where the more ignorant and bigoted members on the commission had raised scriptural objections to Columbus's assertions that the Earth was spherical. Of course there was no such meeting and there is nothing in the Bible about the shape of the world.
There probably were people that thought that the world was flat yet they were a minority, like now we have people that think that creationism is true, or 18% of Americans think that the sun revolves around the Earth (if you are British do not laugh 19% of UK think that).* Yet those crazy assumptions, then like today, had nothing to do with the Bible and had everything to do with personal ignorance.
To understand that the world is round one does not need to know maths, the power of observation is enough. One can observe that when a ship approaches the shore: it appears gradually from underneath the horizon. This is all prove needed. In other words any society with sea travel would know that the world is spherical. Moreover evidence of 'they knew about spherical earth' is found in the majority of art works. Look at the one above. It is Mappa Mundi from a psalter. It is not to be read like Google maps it has little to do with geographical navigation and all to do with navigation through the world view. The map is a circle. Of course it could be argued that it just shows that the Medievals though the world was a flat circle. Then look at the image again. In Christ's left hand you can see him holding the Earth shaped like a ball.This is pictorial evidence that our ancestors were not as dumb as we think they were but then it seems to be the guilt of every generation to think that we are swarter than the previous one. It also shows that art is a historic document that can be used as evidence and should be used as evidence. This (annoyingly) is often forgotten, and lies buried underneath the empty noise of pretentious 'arty' speeches about how 'my life is art and art will save the world.'