A category mistake is an error in logic in which one category of something thing is presented as belonging to another category. For example, to say "the rock is alive" assigns the category of life to that which is not alive. Another example would be to say that an idea is the color blue. It mistakenly applies the category of color to a concept in the mind. That's what atheists do here.
The vast majority of arguments one sees against the existence of God are not really arguments against God’s existence, but the existence of any number of so-called deities which may or may not exist in the universe. This is why, for example, Bertrand Russell’s teapot or the ever popular “spaghetti monster” are so silly as neither have even the basic characteristics of God and, therefore, cannot serve as adequate targets for the arguments raised by atheists. Just as frequently, an assertion of atheism is not the assertion of a complete philosophical or metaphysical position, but a way of saying, essentially, “That thing you say you believe in, which you are calling “God” – I don’t think that thing exists, or any similar thing.”
But in all the great religious traditions, God is not a name for some thing that can have similarities to other things. "According to the classical metaphysical traditions of both the East and West, God is the unconditioned cause of reality – of absolutely everything that is – from the beginning to the end of time. Understood in this way, one can’t even say that God "exists" in the sense that my car or Mount Everest or electrons exist. God is what grounds the existence of every contingent thing, making it possible, sustaining it through time, unifying it, giving it actuality. God is the condition of the possibility of anything existing at all." Therefore, even the claim to reason in the denial of God is, in a sense, an affirmation of God.