Originally Posted by
Ryme
Because at it's core it relies on faith which is, by it's very nature, unprovable and thus inarguable if someone chooses to do so. The best you can is to explain why you think that faith is misplaced, if they still refuse you there's not much more to be done.
As an aside, there are plenty of people that believe in fairies, almost all of them live normal lives - have a Google if you want, the British television company Channel 4 even have documentaries on it which may be free to view on their 4oD service depending on your country.