To be fair, there are more ways than one experiencing what truth is and what things are "real."
We in the Western world especially put a premium on the scientific method – evidence gained from experimentation that is repeatable. Along with conclusions that arrive from unassailable logic without contradictions or gaps.
But this type of knowledge is not applicable when dealing with religious or mystical truths. Whether you call it experiencing God, or having a mystical revelation where you are one with the universe, it is a type of knowledge that is qualitatively different than what we usually experience. It's something that can be hyper-real -- such a perception of reality that is so strong and overwhelming that the normal reality of man-made logic and experimentation fade into the significance beside it.
For those who experience God in this fashion, no amount of logical contradiction will resonate or convince one to deny the experience. Once you personally experience the divine, you know that it is real, know it with a depth of certainty that makes it easy to conclude that those who have never seen it are simply unaware of its reality.
This type of spiritual experience has nothing to do with the rantings of a preacher on the corner. But I think we tend to be a bit too quick to denigrate this type of knowledge as inferior or less worthwhile then what we have come to commonly accept as "knowledge."
From the beginning of recorded history, and probably even further back, individuals have experienced states of higher consciousness and mystical experiences from which they have gained great insight.
"The universe is not only queerer than we imagine,it is queerer than we can imagine."
-- JBS Haldane
I know that all life on this planet is one – trees and horseshoe crabs and people and bacteria are merely the various manifestations that have developed from primal life. Or as Lauren Eisley puts it, if you go far enough back in time you will reach a place where cat and man and weasel leap into a single shape.
I believe this to be true from the scientific record and the power of the theory of evolution.
But I know it in a deeper and to me more meaningful way, because I've "seen" it. It is a different way of knowing things, but does not lack its own form of validity.