Nitpick: If you're referring to the middle ages, nobody there (well, no scholarly person at least) actually thought the earth was flat. That the earth is round was known since antiquity.

The big dispute was about whether the earth was at the centre of the universe; heliocentrism was contradicting the church's doctrine at the time. Somehow this ended up in popular culture as an argument about the earth being flat which it never was.

Now of course, it's entirely possible that earlier cultures, before the Greeks, were under the impression the world was round... or any other shape.