Atheism is a relationship to God.
I can already hear the vehement protests of “actual“ atheists. But honestly, try to define atheism without referencing divinity; it can't be done because atheism is itself a preoccupation with divinity. In point of fact, in my experience the only people who tend to think about God more than theists are atheists.
The person who is not in relationship to God sees no occasion to announce his lack of religious involvement and, certainly, wouldn't care to identify himself thereby. It does not so much as occur to him; far less is it a problem to which he feels "called" to apply himself in the least.
The atheist, however, who approaches such things in a spirit of negation, approaches nonetheless; he is not indifferent. Indeed, he may take them more seriously than many who take them on faith; "for now is our salvation nearer than when we believed." (Romans13:11)
Here, physics and metaphysics part ways: the fact of whether God exists or not is irrelevant. But, then, even among the most advanced mystical thinkers, existence has seemed too narrow a term for God. They have long held that any notion one may form of God is only a remote aspect or representation fit for the mortal mind to contain. A place to begin. As John Caputo’s maxim goes, “God doesn’t exist; God insists.”
For the mystic, the glass is half-full.
The name of God, and the ideas which we are able to form of "Him", however incomplete a measure of what is here, yet rise to an appreciable level. They are not easily disdained. Indeed, the empty space which remains may only lend a greater poignancy to what is able to be uttered.
Let us be clear: for the atheist, God is not nothing, but, rather, an idea which seemingly corresponds to nothing.
Many theologians say the same. For them, perhaps, even more than for the atheists, "God" is just a word. However, they do not deny that, by confronting this particular idea, one is confronting Something; a stab at ultimate meaning. There is something, some event which happens in the name of God.
It is purely subjective, almost semantic, whether we consider such a confrontation sacred or mundane. But, the fact that we take any position at all on the nature of the holy directly involves us in what (to my way of thinking) can only be called "the spiritual life".
Simply put: We cannot confront the idea of God without relating ourselves to ultimate principles; Being, Beginning, End, Infinite, Eternal, One, Many, etc., -- and the most fundamental themes surrounding humanity; Good and Evil, Justice, Mercy, Compassion, Sacrifice, Responsibility, etc.
It is the very essence of religion, or spiritual life, to examine such things, and oneself in relation to them.