Christianity is false, but it is harder to falsify it then it is to show that Barrack Obama is not a non-sapient extinct mammal. I can prove the second false to a five-year-old of average intelligence by showing a picture of Obama and an artist's rendition of a mammoth. It would take some time to explain to the same five-year-old child why Christianity does not make sense as a description of the world.

This difference—that while both claims are false, one claim is much more obviously false than the other—explains why Christianity has many adherents but the Obama-Mammoth hypothesis does not. And we can usually infer from the fact that many people believe a proposition that it is not transparently false, making it more reasonable to investigate a bit before rejecting it.