Well, far from everybody thought so. It was long before this, Pythagoras (580–500 BC) got to the point, the world was round. After him, many people followed – each with own reasons who thought the world was round, even in the Middle Ages. You can check at:
The Bible is not that clear about the world being flat. There are references to „all corners of the world“, but we still use that expresission today although we know better than that. Reason why people believed the world should be flat was Jerusalem being the middle point of earth from which should follow a flat world.