The answer to this question is deceptively simple -- God is what you believe God is.
What? What could that possibly mean?
Simple -- we can divide our "knowldege" into two broad categories; things we "know" from personal experience or education, and those we "believe".
By the time we are 2-3 years old, we all pretty much "know" about gravity. Not the laws of gravity, or the name "gravity"; but we know that if we step off something too high, we will fall. Humans seem to need to "learn" this, whereas other animals, such as cats, appear to have an instinctual knowledge of this.
Similarly, after being burned by hot items several times, we "know" what the effects of hot items are. In a more abstract fashion, we "know" that electricity can kill. Very few of us have actually experienced this capability of electricity. But through schooling, demonstrations of other powers of electricity, etc. we come to "know" what electricity can do. In most cases, we can predict certain things by calculation with mathematics, run experiments to determine whether our predictions are correct, and conduct experiments repeatedly to demonstrate our "knowledge". We can also describe the experiements and expected results in such a fashion that other can run the same experiments and obtain the same results. Once something has been repeated many times reliably, we add it to our body of things that are "known".
However, the things we "believe" are unprovable, untestable, and therefore unknowable. I have yet to hear of a reliable, repeatable experiment that predicts the weight of someone's soul. No one has been able to "prove" the existence of God through experiment. Nevertheless, we use our beliefs as well as our knowledge to help guide our behavior. So our beliefs seem to be as important to us as our knowledge.
Our beliefs and knowledge determine how we deal with other people in society and our families, what we judge to be good or evil (if we "believe" in good and evil), and our relationship with the (dare I say it?) the Universe in which we live.
For me, the Universe is "known". I exist. The people I know and love exist. The planet on which I live exists. All these things exist in something tangible, and for me that is the Universe. God, for me and everyone else, in unknowable. I KNOW of no one who has died and been reborn in any form. I KNOW of no one who has gone to Heaven or Hell. I KNOW of no one who can predict the weight of my or anyone's soul -- or at what time it "ascends" from my body at my death.
Simply put, there is no demonstrable experiment or provable calculation to demonstrate the existence of God. The question of whether God exists or not is UNKNOWABLE. All anyone can do is BELIEVE. And that is the cause of many problems. Many people cannot distinguish between knowledge that is in the belief category from the knowable category. And the subject for another post.