There are always a few people who believe very stupid things. Hell, there are often many people who believe in very stupid things. Flat earth isn't much stupider than crystals and homeopathy in the nonsense stakes.
This idea that in general though, everyone thought the earth was flat at some point is really stupid. The ancient greeks knew the earth was a globe, not only was it accepted as truth it was proved by scientific experiment, Eratosthenes even calculated the circumference of the Earth.
Some people through the ages believed in the disc theory, but probably not nearly as many as who knew it was a globe. A fairly cursory curiosity about the stars and the sun made it easy enough to intuit.