Did people in the Middle Ages think the Earth was flat?

No. Educated people in medieval Europe knew perfectly well that the Earth was round. The belief that they thought it flat is itself a myth, and a fairly modern one. As the historians David Lindberg and Ronald Numbers put it, there was scarcely a Christian scholar of the Middle Ages who didn't accept the Earth's roundness, and many even knew roughly how big it was.

That knowledge came from the ancient Greeks, who documented a spherical Earth from the 5th century BC. Around 240 BC, Eratosthenes even calculated its circumference. It never faded. Dante's Divine Comedy, written in the Middle Ages, describes a spherical Earth with gravity pulling toward its centre.

So where does the myth come from? Largely from the 19th century. In 1828, Washington Irving published a romanticized biography of Christopher Columbus, which invented a scene where ignorant churchmen objected to his claim that the world was round. In reality, the argument in the 1490s was about the Earth's size. Columbus thought Japan was about 5,000 km away; it's closer to 20,000. His critics were right. Later writers, eager to show religion fighting science, repeated the flat-Earth story as fact.

Historians have been refuting the myth since at least 1920, yet it still turns up in popular culture and even school textbooks. The irony is hard to miss: the people who mocked medieval ignorance were spreading a falsehood of their own.