I require this, and personal experience, to know one way or another.
If you haven't experienced something for yourself, then you don't really know about it, you only believe in it.
I'm not suggesting that I care to know one way or another on this topic. It's just that the round-Earthers, and perhaps some flat-Earthers, are pushing their belief as knowledge, when it isn't. I fully admit that my flat-earth belief is just that, a belief.
This ties in with my religious faith: I rejected religions which demanded beliefs, preferring knowledge promised instead by early Buddhism. I suppose that my participation in this thread is more about belief vs knowledge than about the shape of the Earth.