I find it hard to believe that the educated few weren't aware that the Earth is round. For Eratosthenes' theory to be known today,
his book has to have been copied and disseminated. I've read that there was another book in the
library at
Alexandria, which argued that the sun was the centre of the universe. The book was lost, presumably because people weren't convinced, so the book wasn't copied.
What the ordinary people believed was another matter. We don't, of course, have their thoughts on the matter. What we do have is texts like Genesis 1, which describe the Earth as though it was flat. Whoever put Genesis together, put it right next to a second, somewhat contradictory, account of creation, suggesting that they may not have intended every detail to be taken at literal
face value. It may have been accommodation to the views of the people, but we don't know.