How can an atheist be spiritual? Being an atheist means there is no belief in a “god” or “gods”. Are followers of Confucianism and Taoism “spiritual”? Or what about Buddhists? Certainly some Buddhists deify the Buddha in some sense, but pure Buddhism simply holds the Buddha as “enlightened”. And while Confucianism and Taoism are not as well-known, they are definitely “spiritual” without any explicit required “god” belief.
So why do so many assume an atheist cannot be spiritual? Simply because Western spirituality has tended to be intermingled with theism. But that is a bias that is not necessary, as many Eastern worldviews, and even other “primitive” worldviews, demonstrate. So step outside of your theistic / religious bias and see that there are many people historically that have never assumed spirituality requires any “god” belief.
OK, so what is spirituality then? Spirituality is in at least some sense the recognition that there is more than just us. I think those Eastern non-theistic worldviews would agree that is part of it at least too. Maybe some of that “more” is energy, maybe some of that “more” is connectedness. And there is possibly more “more” – I don’t know since it is by definition “more”. And for me it is recognition that there is more than just “science”.
People used to believe that humans and the Earth were somehow special. Now we know that is not the case at all. Our great theories even eliminate the specialness of nearly everything else. Matter and time aren’t even absolute – everything is relative indeed! And yet at the same time those theories keep showing that everything is connected in ways that we do not comprehend. Space and time are connected, and things get entangled.
Spirituality is just the recognition of that “more” that drives “being” and “connections”. And recognition of that connectedness begats morality – of course I care very deeply when we are connected. It is theistic based morality that stagnates and fails to learn to value greater diversity and the world around us. Furthermore, the recognition of the energy and connectedness that is “more” invites us to attempt to engage with it also.