I included the religious argument because ultimately, flat earth is about an exactly literal interpretation of specific scripture. Just for example,
here is one example of a different, similarly literal interpretation of some different lines, with the author taking the position that they teach a spherical earth.
Something to remember is that the debates we're having are old. Very, very old, older than the new testament. And as with all things old and religious, there is only persuasion and interpretation of the text; observation and experiment don't really influence religious thought. And that's fine! Whether or not Jesus rose from the dead is not really a matter of experiment; the book says so, and you believe it or you don't.
The shape of the earth is a different story. Humans can in fact use considerable cognitive abilities to learn about the natural world around them, and at every turn, new discoveries about the world have divided the devout. One can wonder at the magnificence of God's creation and the elegance of galactic orbits, the mathematics of quantum physics, the complexity of space-time, etc., or one can insist that these discoveries are wrong, that the old way was right, that Feynman, Einstein, Faraday, Newton, Copernicus, and Democritus were all wrong or liars or worse, that the earth is flat and the universe is geocentric and that heaven is an actual, physical place above the dome of the sky. But in this case it is not old; that is, we can see for ourselves today if these interpretations are correct.
After all, the book never does explicitly say "The Earth is flat," though it does say things that can be interpreted to support this conclusion. These same things also hold if the Earth is understood to be a sphere. And because we can observe, all over the world and in all manner of ways, that the Earth is a globe, that the Earth orbits the sun, that it has an axial tilt that causes the seasons, etc., the only correct interpretation is that the book is referring to a round Earth...
because, for the nth time, the Earth IS round. I know just saying as much doesn't change any minds, and I actually couldn't care less. Saying it's flat does not make it flat, and saying it's round doesn't make it round; its observable nature shows that it is round.
If one thinks the book says the Earth is flat, while the Earth obviously is round, and the book is by definition correct, what must be true is that the thinking is incorrect, and the book says the earth is round.