Well-intentioned religious folks ("WIRFs," an acronym I just now coined) are fond of suggesting that agnostics and atheists like me should act as though god† exists, just to be on the safe side. If the WIRFs are right, the argument goes, we doubters will be spared eternal torture, and if they're wrong...well, no harm done.††
Is belief in a myth (or acting as though you believe in a myth, which amounts to the same thing) harmless? Let's consider a practical example. A 5-year-old who believes in Santa Claus is cute. A 15-year-old who still believes in Santa Claus is a matter of some concern. A 35-year-old who sincerely believes that Santa Claus exists (as a corporeal being, not just in the "Santa Claus is the personification of the warmth and generosity of the Christmas season" sense of "existence") would be viewed as needing the services of a mental health professional. Even if belief in Santa Claus doesn't lead someone to harm themselves or others, we'd rightly wonder if they're in touch with reality and can be entrusted with grown-up tasks.
The biggest danger of belief in an incorrect proposition is that wrong information can lead to bad decisions. If you believe that you're going to be whisked away to paradise at any moment, you might not bother to make long-term plans like saving for your retirement or caring for the planet. If you believe that childhood vaccinations cause autism, you might decide not to get your kids immunized against totally preventable but highly contagious and occasionally fatal diseases like pertussis ("whooping cough"). If you believe that a girl who's been raped has "lost her honor," you might consider it necessary to punish or kill her "to restore her family's good name." GIGO.
None of us is completely rational; we all have some beliefs that are based on emotion rather than reason. That's part of our charm. In the interest of making good decisions for ourselves and those around us, though, we should at least be willing to examine our beliefs, ideally by discussing them with others. Just as the best way to learn a subject is to teach it, the best way to figure out what you believe, and why, is to explain it to someone else.
† The god the WIRFs believe in, of course, not those heathens' idea of "god."
†† This is often referred to as "Pascal's Wager,"††† after French philosopher Blaise Pascal.
††† After I'd started composing this post, I noted that Greta Christina is also taking on Pascal's Wager. I'm waiting to read her post until I've finished mine so I won't (unintentionally!) plagiarize her. I have no idea what she wrote, but knowing Greta, it'll be good.