Below, Bill Nye handily explaining WHY it's important not to simply "let them be" regarding creationists, flat-earthers, etc.
There are two kinds of people in the world: Thinkers and Believers. The distinction has nothing to do with religion or "atheism" or even intelligence - it's about how you approach life on a day-to-day basis. Do you think for yourself, or do you let someone (or someTHING) else decide for you? Do you put your trust in "traditions" or do you apply logic? Do you "feel" or do you reason?