There are different kinds of ‘knowing’. Some people derive convictions from extra-sensory faculties, like intuition or convictions that stem from feeling, not intellect.
As the heroine was asked in the story ‘Contact’, how do you KNOW you love your wife or children? Can you prove it?
True, but there's a fine line between intuition and delusion. A person can believe something so strongly, that for them, that thing becomes 'real', but it's only in their mind.
As a long time seeker of personal enlightenment, I can say that I'm very familiar with this phenomenon, so I don't just discount the spiritual realities of others. Just because I can't perceive a thing does not mean it doesn't exist. It just means that I'm incapable of seeing it, or it doesn't exist. I'm willing to allow for either reality, but I can only operate on that which is real to me.
Now, there are also those things which I feel I comfortably know, and which I have great certainty on. Some of those things are outside the norm of everyday 'beliefs', and are part of what we might call the spiritual realm. I *know* these things because I can perceive or communicate with them in some tangible way, even though many others can't.
In the same vein, there are some *truths* that others hold as real, that I'm not capable of perceiving or communicating with. For me, those things are not real, and I won't accept them just because another person claims to have deeper insight or powers of perception than I do.