Well, that's not strictly true. I mean, most people I think, have quite a good belief in things like colours, because they actually look like they're out there in the world, regardless of whether they are or not. And it takes quite a bit of work to actively disbelieve in the existence of colours (particularly inasmuch as colour is useful for all sorts of things). Disbelief isn't the default position if you have a perceptual framework that mandates or encourages certain kinds of belief. I think that's of particular importance with religion, because, for a lot of people (possible even most people) religious belief will be the default position because, well, that's the position you're (rightly or wrongly) starting from, and operating against.