Well first of 'we' didn't believe anything because 'we' were still one thousand years away from being alive. Second of all, the world didn't believe the Earth was flat, in fact after there was a lot of optimism in that field at about 400-300BCE with Aristotle noting how the stars set, and putting forth his theory of a spherical Earth. Of course, the year is 300BCE so the message got across to roughly eight people at the time within his own family. Then there was another ancient Greek, I've forgotten his name, who figured out in 200 or so BCE that in order for the Earth to be flat, the sun's rays would have to be parallel to the Earth, which they weren't. There were many mathematician, scientists and the like coming out with these claims until the first century BCE. Then there was the whole Church controversy with the Roman Catholics silencing anyone who didn't believe in St. Augustine's view of antipodes, citing religious theories dating back to Adam to prove his point. Then there was nothing until Copernicus and Gallileo in the 1400-1600s and even then, this was still a very new subject. The different between us now and them was that this was groundbreaking stuff they had to really work toward, now we can Google search the circumference of the globe in less than a .14 seconds (that's how long it took my computer, apparently). But if you ask someone to actually figure it out without a computer now, I'm betting aside from obvious professional, many of us wouldn't know how and where to start. So just because some people believed the world was flat, doesn't mean they weren't smart. The works of Aristotle and the Greeks, of the Early Middle Age philosophers was highly thoughtful and quite solid material. I don't think there are many nowadays who can match that sort of intellect in context to the age they live.