It's actually a myth that the Catholic Church or medieval folk believed that the earth was flat. After the spherical nature of the earth was first proposed (and the radius relatively accurately measured) by the Ancient Greeks during the classical period, pretty much no educated people in 'Western' civilisation believed in a flat earth. This includes the vast majority of religious scholars and clergy. That people in the Church or during medieval times thought the earth was flat is an idea that really only caught on during the 19th and 20th centuries.

However that didn't mean that the whole world thought the same. For example Chinese cosmology and astronomy maintained a view of a flat earth until the 17th century.