Basically, the earth is a flat disc protected by an invisible barrier called “the firmament,” a dome-like feature that also happens to be referenced in the Old Testament so you know it’s real. Copernicus, of course, fucked everything up by introducing his theory of heliocentrism. Flat Earthers acknowledge that while Copernicus’s math was right, he lacked the technology necessary to definitively prove his theory. Still, the idea took hold, governments started preaching it, and a centuries-long lie was born.