Well there's just an itty bitty problem here.. Belief is often a matter of preference, the same as a person's opinion is. So it seems to me the same question applies to someone's opinion, can you prove it? What do you actually think can be proven anyways? When you follow the logic back to the basics don't you have to make some kind of assumptions to start with? If you really want to get down to it most people have some kind of beliefs that way - I don't think it's all that meaningful to distinguish between religious and non-religious ones. Maybe we can make the distinction between people who acknowledge this and people who don't, though. After all, if we're going to talk about mental illness a person deluding themselves would qualify, no matter what it is. More than one religious text (including the bible if I'm not mistaken) says something to the effect of if you think you know something when you really don't, you're deceiving yourself. Of course even though the logic is sound, many of us might have issues with the bible in particular and religious sources in general, just don't like the source. We wouldn't want to exacerbate that now.. Say, might that be mental illness?