By 'theism' I mean the hypothesis that God is the creator of the physical universe. I take the word 'God' to be a title that, by definition, can be borne only by a perfect supernatural person. While some have dismissed this conception of God as religiously insignificant, I am convinced that, for millions of Jews, Christians, and Muslims, factual belief in a supernatural person is essential for making sense of their forms of worship. I mean the hypothesis that the physical universe is a 'closed system' in the sense that nothing that is neither part nor a product of it can affect it. Thus, just as supernaturalism is broader than theism, naturalism is broader than atheism.
Someone can be an atheist without being a naturalist.
In other words, someone could believe in the supernatural without believing in God. I have chosen to adopt the label of a naturalist in order to emphasize that I do not believe there are supernatural beings of any kind.