Knowledge? There is none whatsoever, IMJ. (I assume you are using that upper-case "S" on purpose.)
I think you mean "belief", or perhaps "experiences that have been interpreted as Spiritual".
I do not doubt that there are unusual experiences one might have that lend themselves well to grand interpretations. One might hear a voice in one's head that is not one's own and think that this is God speaking, or an angel, or a demon. But the brain is not so uncomplicated to jump to those kinds of conclusions. There is much going on in the brain that is not directly tied into one's conscious experience of "self".
I've never heard voices, but I once had an experience where I was working on a math problem, and I had decided to shut my eyes and rest for a few moments, thinking of nothing in particular. Suddenly, before my mind's eye I saw a green phospor vector diagram with the correct answer to the problem. It was a complete surprise to my conscious "self".
Now, I could interpret that as the God of Mathematics, or maybe one of his Holy Grad Students, giving me a divine revelation! But it is more in the spirit of Occam's Razor to conclude that it was some subconscious process in my psyche that had worked on the problem and given me a hand.
Intepretation is a very important factor in all of these claims that humanity had somehow had knowledge of something "Spiritual". They certainly had experience with something, but maybe that should be a lower-case "spiritual" instead.
Personally, I consider myself a spiritual atheist, but I don't intend that to mean that I believe in supernatural or disembodied entities. I mean simply that there are desirable vantagepoints to have on reality that involve a way of being that is not like the sort of experience of day-to-day life that most people have.
