The answer is, they didn't. I am just messing around about how a space faking Illuminati conversation went down. The fact is that no civilization in recorded history seriously believed the Earth was flat. Some flat-Earthers will claim that this is a 500-year-old conspiracy but they do like to conflate the dawn of the heliocentric model with the flat Earth conspiracy. That's just how they roll. If you ever encounter a flat-Earther and "debate" them -- which will consist of you stating facts and they ignoring them -- they will use Question 1 from the Flattard Playbook (page 1 of 1 huge font, double line spacing), show me curvature. There are many examples of curvature, Earth from space, of course, but they will dismiss that as "CGI" or "Fish-eye lenses" because reasons. Lake Pontchartrain and the Turning Torso Youtube videos of curvature are pretty damn compelling. Of course, they will dismiss that as "perspective" or outright refuse to look at it and deny it. The personal incredulity is strooooong in them. Powerful flattards they have become. Flat earth anonymous reptilians (f.e.a.r). - Hehe, I might just have to use that one time, nice.