There were very few things anyone could say about religion that irritated me equally both as a Christian and as an atheist. Answering “I'm spiritual” to “Do you believe in God?” or “What is your religion?” was definitely one of them.
Whether I was a Christian or an atheist, I saw it as one of the most entrenched forms of namby-pamby, testicle (or ovary)-deficient fence-sitting ever invented. What the hell does it even mean? As an atheist, I always took it to mean, “I'm an atheist, but too scared to admit it”. As a Christian, I took it to mean, “I'm a heathen who likes smoking pot”.
To my way of thinking, if you were a non-believer, just say so, without any bells and whistles. As a plucky, street-fighting atheist arguing with semi-Catholic gays in Spain, I would say, “Do you believe that a virgin had a baby? A big boat was manned by an old man and every animal on the planet? That slavery is justifiable?...No? Then you don't believe in Christianity either. No me jodas.*” This would inevitably lead to exasperation or aggravation on the part of my interlocutors, and smug irritation on my part at such rampant ignorance.
Good thing I wasn't at work, because they would've probably fired me.
I say that because I have a very strange job which requires me to listen to what people say in multiple languages, eloquently and less so, and rapidly get to the bottom of what they mean in spite of the words they may use or misuse. I'm not bad at my job, and that part of it at least is one of my natural talents. But for some reason, that talent deserted me for years when talking about spirituality.
It's true: there is no evidence that spirits exist, in the strictest sense of the word. Casper the Ghost is a cute, cartoon character, but nothing more. Thunder and lighting seem to be caused by colliding air fronts rather than angry, intangible patriarchs. It follows that the word “spirituality”, in the strictest sense, is similarly nonsensical. But what Atheists often fail to realize is that what the word “spirituality” represents in common usage, is very real and very valuable to many if not all people.
The way I have come to understand it, spirituality actually means a feeling of connectedness to the universe and/or gratitude for the privilege to exist. Before my Atheists get their knickers in a twist, I'm not referring to gratitude towards any actual being. The key word here is feeling.
For some time, we have known that all lifeforms are carbon-based. We are intellectually aware that our sustenance in food, water, and air comes from the environment around us. We also realize that genetic differences between humans---across sex, race, or any other fictitious category we have invented---are infinitely small when looked at in absolute terms. But in the developed world, food of any kind is seemingly constantly and conveniently available, dead and pre-packaged for no work and a modest price. Racial and national differences determine whether you can become a banker or a sweatshop worker. We pollute and use the planet's resources at an alarming and unsustainable rate as a way of life. We talk about nature as if it were a separate entity as opposed to something we are part of. In its current state, our world obscures the fundamental truths of our existence and turns reality on its head. This gaping dissonance between what we know to be true and are often led to believe and feel is not healthy. Ergo why the form of psychological abuse known as “gaslighting” is particularly insidious. In order to recover some semblance of balance and healthy functioning, it is not enough for people to know the true nature of their world. For better or worse, humans are both rational and sentimental animals. So they usually have to feel it as well. That feeling is probably beneficial to anyone. The problem is that organized religions tend to hijack and distort it (i.e. the "Jesus-shaped hole in your heart" that Evangelical Christians love to tout). On the other hand, many atheists often simply deny it and imply that anyone who is not brain dead should be able to get by on intellectual understanding alone.
So I've changed my arguing tactics. To the atheists I say, “Ever felt wonder when looking at a star-covered sky or a forest? Ever felt peace when seeing a rainbow after a storm? Ever smiled upon realizing you come from a long line of other human beings that stretches through time and space? Then you are spiritual.” To the believers I say, “Ever met an atheist who feels the same wonder watching the sky or being in the forest that you do? Or the same peace when seeing a rainbow? Or smiled when thinking you come from a long line of other human beings. Then why do you think that feeling is exclusive to adherents of your particular mythology? You're spiritual, too.”
No me jodáis*.
*A fairly common expression in Spain which means “Don't bullshit me”, in exactly that register.