Even the ancient Greeks knew the world is round - check out the statues of Atlas with it on his shoulders
They calculated its size and modern science shows they weren't far out.
Anyone who looks at a horizon can see its curved!
The argument against Columbus wasn't that he would fall off the edge, his opponents had calculated (correctly) that he would run out of food and water before reaching his destination China.
He ignored that and luckily for him and his crews collided with Cuba or wherever.
The notion that Columbus was an enlightened, modern, 'round earth' man and his opponents superstitious and ignorant priests was only invented in the late 19th century by the american writer Washington Irving (Sleepy Hollow, Rip van Winkle) in his biography of Columbus.
Irving had an anti-'old world', anti Roman Catholic church agenda which probably explains this fiction. But his book was widely read and the notion stuck that pre-columbus people thought the world was flat.