The ancient Greeks knew that the Earth was round, and had correctly estimated its circumference to within a few percent. 
Medieval Europeans knew this, and had correctly warned Columbus that he would never get to India with his limited supplies. Columbus relied on a fallacious argument that the Earth was much smaller than the Greeks estimated.
Some writers have presented the mistaken idea that medieval Europeans used a literal reading of the Bible to conclude that the Earth was flat. They supposedly tried to convince Christopher Columbus that he would fall off the edge of the Earth, the story goes, but Columbus proved that the Earth was round by discovering America. 
- The Sum of Existence, by A.R. Barnes, Jr. - contains an account of how Eratosthenes estimated the circumference of the Earth.
- "Among various rude tribes we find survivals of a primitive idea that the earth is a flat table or disk, ceiled, domed, or canopied by the sky, and that the sky rests upon the mountains as pillars."
- "Eratosthenes erected a prototype of ancient Egypt and placed two vertical sticks, one in the position of Syene and a second in Alexandria. He studied his model judiciously as he considered all possibilities. Plainly, he reasoned, the solution had to do with the shape of the Earth. He asked himself what shape the Earth had to be for a stick not to cast a shadow in Syene, but to cast a shadow with an outward angle in Alexandria? Eratosthenes knew that the sun was very far away, and as such, once its rays reached the Earth, they would hit the ground parallel. Therefore, if the Earth were flat, then both sticks would either cast no shadow or they would cast similar shadows. If the Earth were concave, then both shadows would curve inward, one toward the other. Once he contemplated all alternative shapes, Eratosthenes settled upon but one logical conclusion: the Earth was round." The Sum of Existence, by A.R. Barnes, Jr.
- Saying "to the ends of the earth", "the four corners of the world" or "the sun sank into the sea" does not make you a flat Earther.The Myth of the Flat Earth (Bede's Library)
- "Christianity did not invent or promote the myth of the flat Earth." Who invented the flat Earth? (ChristianAnswers.net)
- The Myth of the Flat Earth, by Jeffrey Burton Russell.