Flat Earth belief tied to Christianity is an extremely recent thing, relatively speaking. The Catholic Church, which represented the massive bulk of the religion for a millennium and a half, has always maintained that the Earth is spherical. I don't think the Bible was used as justification that the Earth is flat until the 19th century, and that only by a small number of extreme fundamentalist sects.
It is, of course, a myth that it was the prevailing belief that the Earth was flat until Columbus discovered America.