This debate is usually expressed, by those on both sides, as a debate as to whether God exists. So to move (as I have done in my life) from believing in God to being an atheist to believing in God again is to start by thinking that some entity called “God” exists, then to think that no such entity exists, and then to go back to thinking that an entity called “God” does exist.
However, Thomas argues that we cannot talk about God “existing” in the same way that created things exist. God is existence. Similarly, God is goodness (not just “good”), and he is love, and so on.
In other words, the debate between atheism and theism is not a debate about whether God exists – a debate that can, and does, swirl around fruitlessly for as long as the participants can summon up the will. Rather, it is a debate about the nature of existence.
As the Canadian philosopher George Grant put it:
What is given us in the word ‘God’ is that goodness and purpose are the source and completion of all that is.
In other words, faith in God, as Christians understand him, is a claim that existence is fundamentally personal; that goodness (a word which here encompasses truth, beauty, love and all the rest) and purpose are hardwired into reality at its most fundamental level, in the sense of being both its “source” and “completion”.
Atheism, by contrast, is a claim that, at root, existence is fundamentally impersonal. It’s not that atheists deny the reality of “goodness and purpose” (though some do), but to be an atheist is to deny that goodness and purpose “are the source and completion of all that is”. Rather, they are something that we as human beings create for ourselves, a shaking of the fist at a cold and indifferent universe – an image not without its attractions, but a very different conception of reality from the one that “is given us in the word ‘God’.”
This also suggests that the difference between atheism and belief in God is ultimately a matter of competing aesthetics rather than logic.