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.
Well, yeah, because it demonstrably works.
But this type of knowledge is not applicable when dealing with religious or mystical truths. Why not? 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. Qualitatively different is one way of putting it, I suppose... 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. It may feel real to the one person experiencing it, but what goes on in one person's head has no bearing on actual reality.
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. It may feel real to the one person experiencing it, but what goes on in one person's head has no bearing on actual reality.