We didn't, most people were aware the earth was round and orbited the sun thanks to the Ancient Greeks and their science. It was just the church trying to push onto everyone that the earth was flat, wiki has a good article entitled 'flat earth myth' which debunks the whole crap about everyone believing the earth was flat. Very good article, worth a read.
So the Bible and Quran do not depict a flat earth? Surely this is why the Church or Muslim scholars would argue FOR a flat earth. The article says scholars did not believe, doesn't mean regular people did not believe, also I think the Church was a major institution at the time so what they said was probably more or less the truth for people.
As a small side-track, lol at this article:
http://www.answering-christianity.com/earth_flat.htmThey criticize the bible for having a flat earth depiction when the exact same phrases can be found in the quran.
Also, you don't think the modern Catholic Church would want to downplay the silencing of scientists during the Middle-Ages? Not only in regards to the flat earth but also the model of a geocentric universe.
I mean if we look today, at religious institutions, were do they stand in terms of Evolution? What is their explanation? Creationism? They are not persecuting anybody sure, but what about their believers? What are they told to believe? How do you think people will view this Evolution vs Creationism debate 200-300 years from now? (If anybody from that time is reading this now please unfreeze my body, I'm afraid somebody might have forgotten it, if I am reading this then have I just destroyed myself, in the style of Back to the Future? If so then why I am still writing? This means that I either don't exist in the future, or me reading this in the future won't matter.)