The ancient Mesopotamians believed the Earth to be flat. Pythagoras was one of the first to theorize about a round Earth, and Columbus popularized the idea. Ultimately, Flat Earth Theory was revived by Samuel Rowbotham.
FES holds appeal to religious groups because it makes the Earth more significant than other planets, and coming up the a natural creation story for the Flat Earth universe is next to impossible. However, as you may have noticed, the vast majority of debate on these forums ignores the possibility of a deity entirely. A Flat Earth and God are not necessarily related.