It could mean that I'm a nice person. I pet dogs, kiss babies, and help elderly folks cross the street. I pay my taxes. I believe in God or something equivalent. It might even mean I pray or meditate. I respect (or not) all faiths or at least see them as means to the same end. (BTW: is the term "organized religion" an oxymoron?)
But I wonder how many of the millions who place themselves in this category really live a life of daily prayer and meditation, self-sacrifice or ego transcendence in the name of enlightenment or other divine goal, or service to humanity as an act of devotion. It's true that few religionists do any of these things, either!
What I've described is more what most people might imagine the life of a monk or nun might be like and how few of these there are in the world (even among monks and nuns!). And I think that's my point.
At least a religionist participates, however wanly, in his faith and commits himself to serving its cause, attending its religious services, and giving in monetary support. By contrast, being only spiritual but not religious might mean one does nothing at all! Maybe he sips a latte on Sunday morning while reading the proverbial (digital) newspaper on the deck in the sunshine!
The fact that many of us feel religion has abdicated its true calling by becoming partisan, sectarian and promoting divisiveness rather than peace and harmony is wholly and truly understandable.
Nonetheless, an objective reading of history will also disclose that religion has also been a force for unity, harmony, respect, rule of law, and peace at key moments during human history. No other human activity or impulse has this power. Legislation, imposed by police force, is insufficient. Reason has limited power to change behavior. High ideals and spiritual consciousness as exhibited not by prelates and popes but by saints HAS this power.