Earth is not flat and we didn’t believe it was flat in the Middle Ages. I doubt the vikings believed it and in fact we know that at least as far back as the ancient greeks, people knew the Earth is sphere-ish.
Why do people believe that people used to believe the Earth was flat?
For instance, the image above, the Flammarion engraving, is from 1888. That is a bit late to be from the Middle Ages and there is no reason to believe that it is based on anything earlier.
I can’t explain why some people believe the Earth is flat today, but there may be a reason why so many believe that it was a belief before.
But first, how do you know the Earth is sphere-ish? Well, there’s the “Even a Kid Can Prove the Earth is Round: Here’s How” article, which does require that that kid is able to travel large distances, but still, it is simple. It also mentions the ancient greeks and the fact that they calculated the size of the Earth to within 10% of its actual size. There’s also the “10 easy ways you can tell for yourself that the Earth is not flat” article.
My favourite story about the round Earth is that no-one argued against Christopher Columbus, believing the Earth was flat. No, they argued that he couldn’t possibly carry enough provisions to reach India going west. They were right – they knew how big the Earth is – but Columbus didn’t believe them, because he had made his own calculations and, due to a mistake in a translation, believed the distance to be about a third of what others believed. Everyone knew the Earth was round, Columbus just believed it to be smaller. Lucky for him, and not so lucky for lots of other people, America turned up at about the right distance for where he thought India would be.
But I think this article on Answers in Genesis explains why people believe that other people used to believe the Earth was flat.
It also explains why the Bible does not say that the Earth is flat. Answers in Genesis is a creationist site, but it’s very interesting and I would say that their explanations of what is actually said in the Bible are quite good. Pity they are wrong about evolution and geology.