Aristotle, Socrates, Ceasar, all people who lived thousands of years in the past. I'm talking about scientists (not philosophers) who lived in relatively recent times, especially around the time of your book. People who contributed to the current level of scientific advancement. How could they get everything else right and yet not realise the world is flat? It's a bit of a no-brainer.
It's also a bit weird for Christians to criticise the Roman beliefs. Yeah, they were all fools, believing in invisible gods who can do magic. Not like Christians believe in anything like that eh?