Knowledge for humans comes from two sources — firsthand and secondhand. Firsthand knowledge is direct experience of a thing. Secondhand, when it comes down to it, is communication of firsthand experience. We can get more complicated if we like by measuring links in a chain, as in the phone game, but for practical purposes, we can look at it simply. Any communication I perceive is secondhand knowledge if it is part of an unbroken chain ending in firsthand experience.
Note that indirect empirical evidence still can count as firsthand experience. In this way, a scientist who notes the existence of electrons by observing the results of an electrical discharge has firsthand experience of electrons.
Consider Saturn. I have firsthand experience of Saturn, since I have looked at it with my own (telescope aided) eye. I can tell you what I’ve seen, and I will be giving you knowledge through communication. Of course, it is up to you to either believe me or not, and this is where empiricism is so valuable. You can buy your own telescope and find Saturn yourself, and verify the secondhand knowledge with firsthand experience.
With god, there’s no endpoint. Though many people claim to have experienced god, there’s no scientific endpoint where someone experienced empirical, reproducable firsthand knowledge of god. There’s just an unending chain of people with secondhand belief.
This is a bigger problem than it may seem. Let’s return to Saturn to illustrate it. Suppose you have never heard of Saturn. I can tell you that it is a gas planet encircled with rings made up of tiny rock or ice particles. At this point, you have a rudimentary understanding of what Saturn is, but only because you have firsthand knowledge of each of the concepts. Earth is a planet, and you have firsthand knowledge of it. The air you breathe is a gas. You’ve worn or held or seen rings. You’ve held or walked on tiny rocks, and you’ve certainly experienced ice. Each of these concepts is meaningful to you because of firsthand experience.
Even if I get a little abstract and tell you about the superheated core of Saturn, you still have a direct chain of knowledge. Though you’ve probably never seen a ball of superheated gas, there are scientists who have, and they’ve made it quite plain how you, too, could become a scientist and see what they have seen.
You can see what I’m getting at. At each step of defining something which you have not experienced, I must use words which refer to things you have experienced. There’s always got to be some kind of link to the empirical, scientific world.
With God, you can search for your entire life, and you will not find the endpoint. There will never be anyone who has direct knowledge of “supernatural” or “all powerful” or “spiritual” or anything else.
It’s important to realize that many people believe they have experienced these things. When they think of “supernatural,” there is a concept in their brain. The problem is that in the empirical world, they are not referring to the thing they think they’re referring to. When a person says they’ve had a supernatural experience, what they really mean is that they experienced something that seemed magical or indicative of something, but the only direct experience we can observe is that they have had a physical experience which is part of the natural human experience.
To focus on the main point, however, we must note that the words we often use to describe god do not have any actual empirical content.
God is a being. Ok, except that a “being” is a physical conglomerations of atoms that has the property of “life” which is also a physical process. Since God is described as non-physical, neither his state as a “being” or “living” has any meaning. As we should all be aware, saying what something is NOT does not define a thing unless we have a universe of discourse. There is no universe of discourse for “that which is alive while simultaneously being not physical.
God has power. Ok, except that power has meaning, and all power is material. Power is a measure of energy transmission. So, does god have electromagnetic, chemical, kinetic, or nuclear energy? What kind of energy does god produce? How does he produce it if not through physical means? If the form of energy is not one identified by science, what is it? Energy is identified by the work it does. That is, we knew to look for electrons because we saw lightning, and felt our hair stand on end. We knew to look for gravity because things fall. What is the physical evidence which demonstrates the existence of an energy which science has not yet identified?
With any word associated with the definitions we use for God, we run into the same problem. Either god is material, or the words have no meaning. If the words have no meaning, then we have no definition. We have words which appear to make sense because of their real meaning, but which have no meaning because they are being used to literally describe things which are not the real meaning.
In the end, there are two “ultimate” problems with defining God.
1) There is no empirical endpoint to secondhand accounts of God.
2) There are no words which have real meaning when applied to anything outside of the material universe.
When we put these two facts together, we come to the inescapable conclusion — Once we properly understand the undefined nature of God, we realize that atheism, properly understood, is literally the rejection of nothing at all. To say that an atheist rejects the god concept is just a more polite way of saying they reject that which utterly lacks definition or coherence.