Sorry if this is sloppy for I am very tired. I am writing this because an impulse has taken over me and wills me awake in order to write this. First of all, I should make clear this is not about existential stuff but what it means to exist.
For me, to exist always meant to have physical characteristics. Even thoughts for me had a spatio-temporal property. I aware that other people conceive of existence differently than I do, but I saying, it is so fundamental to my understanding of reality this meaning of existence.
I am a very frontal person. This probably has something to do with the damage my brain received from a fever during my birth. When I was seven, brain scans revealed that part of my frontal cortex was as active as an adults. This may have something to do with my view that reality is something you can touch.
Like most children, I can to know about the world by touching and exploring what happens to objects. For instance if you want to lift an block you have to use your arm or some substitute in order for the block to move. My whole world view is shaped by these rather simplistic hands-on interactions. This is probably why as even a young child the whole idea of a deity made no sense to me. How could a deity lift that block without using its hands or any other kind of contact? Deities are incomplete concepts to me for they said to have to ability to do something but are stripped of all the means in order to actually interact with the world. It is like asking a guy without legs to run.
[Edit: pardon this short tangent, for I will return to the argument of this paper by the end of this explanation of my naturalism.] This physicalism even extends to such complex things as emotions. For me, emotions are a kind of information that thinking beings can synthesize. This emotional information can be coded into music and the like for thinking beings to receive through the senses. When a organism expresses something with sound, the information quickly disintegrates due to the dangers of the sound medium. For crickets, the hope is that the chirps will conveys its message to a mate before being destroyed.
All biology reflects this conflict between information and chaos. DNA needs to be protected and capable of replicating itself. Natural selection forces evolution upon the DNA in order for information to survive and compete in an increasingly diverse biosphere.
Humans have taken information two steps further by collective learning. Instead of the knowledge being stuck within the organism. Complex thoughts and lessons can be passed from generation to generation, not simply by word of mouth but by a much more resistant medium, writing.
The only reality I understand deities as having is that of ideas. Like all ideas, they spread from central nervous system to central nervous system. Religions particularly go through natural selection and evolve through time better ways of replicating. Because the nature of human thought, religions have to adapt to the human brain. The human brain has intuitions like those of logic and it has reliance on direct experience.
Since supernaturalism is incompatible with my intuitions, I am an atheist.