Thomas Huxley coined the word agnostic as a play on words. He was a philosopher who was irritated about the metaphysical presumptuousness of the philosophers around him who all seemed to know the secrets of the universe as though they had some special knowledge about things no one can really know about. He compared them, derisively, to the gnostics of the early Christian church. The gnostics were a sect of Christians who believed they were in on secrets that Jesus gave to a handful of his disciples but not the others. Gnostic in Greek is one of the words for “to know” and so the “gnostics” were those who thought they had “special knowledge.” It’s basically like “those in the know.”
SO, Huxley compared overly presumptuous metaphysical speculation and confidence to the gnostics, a sect believing it had secret knowledge. He contrasted himself to them by calling himself an “agnostic” (a “not gnostic”) who had no special knowledge about the secret metaphysical/theological truths of the universe.
So, the word agnostic has grown to mean a position of confessed not-knowing in almost any area of disputable beliefs. But primarily it applies to the theological position of declaring yourself as not knowing whether or not there is a God. Some Christians like to distinguish two types of agnostics: those who say THEY just don’t know if there’s a God and those who claim NO ONE can ever know whether there is a God. Clearly Huxley meant the latter. He meant to claim such matters were inaccessible to human knowledge and to have knowledge would require a preposterous “secret knowlege” that no one should feel entitled to claim themselves a right to.
Yet, there is still humility to agnosticism. It’s not audacious enough as to declare knowing that there’s not a God but it is saying that such questions are unanswerable and left alone. It’s not an opening for others to say they believe anyway. It’s not an outright claim there is no God. It’s a position that says we should all admit we know nothing about such things.
Atheism is just the firmer claim there is no God. I wish it wasn’t so closely linked to the attitude you described being wary of whereby someone claims that all knowledge is scientific knowledge. That’s scientism. That’s the (naive) belief that science can answer every question. I (and Nietzsche incidentally) completely reject that way of thinking. I think science is our most powerful and compelling mode of knowing and I think that it is a model for its insistence on method and experience and verifiability and falsifiability as tests for knowledge.
But, ultimately, science cannot answer many metaphysical questions that I think we can formulate relatively defensible beliefs about. Neither can science say very much at all about values and ethics. And again, I think there is much to say. Essentially, there are many topics for philosophy and for the social sciences that require modes of inquiry that are messier than science for being less quantifiable, but nonetheless are valuable forms of inquiry.
Just theology is not one of them.
So, you can be an atheist like me and Nietzsche without adopting scientism.
I’m technically an agnostic. I believe we cannot know the source of eternity in the universe. All we know is that in some way something must just exist. Whether that’s an eternal character to the stuff of our universe or whether it’s a seperate being is an unsettlable question. I’d rather not answer it therefore. But, if pressed to give an answer, I would say it’s a simpler and therefore less presumptuous answer, to simply say there’s something eternal about the world we do know rather than make the huge unwarranted leap to posit an entire other being that we can not know.
Ultimately, I call myself an atheist because due to my agnosticism, I de facto live like an atheist and I have enough antipathy towards religion and monotheism that I like to express it in the least compromising title available. It is also a matter of importance to me that we deliberately accept a godless universe and pursue reframing our values in light of that apparent situation. The term atheism is more consistent with my insistence on such an attitude.