When the theist uses the word spiritual, he either means having to do with spirits like gods and angels, or he's placing fanciful speculation in the unfalsifiable on an even or greater footing with critical thought and empiricism, which he scoffingly calls materialistic, myopic, and scientism.
My position is that reality (nature) is physical. It comprises energy, matter (and form), force, space, and time. Man. like everything else, is made of these. Everything that exists does so in space and time and interacts with other existent things, and that to say that something is undetectable by all detectors in all times and places is to call it nonexistent.
How about you start by giving a good of definition of how you mean the word spiritual? I ask because I don't believe that most people using the word mean anything more specific than what I've described, and you're looking for how to reach the skeptic. This is how. Be clear. Support your claims with evidence where you can. Make sound arguments and offer rebuttal, by which I mean falsifying counterarguments, not mere dissent with handwaving, ad hominem, or what you believe instead without rebuttal.
Let you offer mine. Spiritual for me refers to the euphoric feeling of belonging and connectedness, oven accompanied by a sense of mystery, awe, and gratitude. It's antithesis is a sense of alienation (source: pink Floyd*). I can occur while gazing mindfully at the night sky, when a drop of starlight impacts the retina and contemplates how far it has come over how long and how we are related to it being made of stardust. It comes when transported by a passage of music, a stunning sunset, or during a good belly laugh. I can occur while gardening mindfully as one contemplates the connections between the flowers, pollinators, and birds and one's own connection to them. All is well in the world during these moments.
That's the spiritual experience - one I once mistook for the presence of the Holy Spirit thanks to a charismatic pastor, my first, who could make the room have this experience. It's an endogenous experience generated de novo in the brain frequently mistaken as something external being experienced.
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* These are all the opposite of the spiritual experience, of connectedness and belonging:
[1]
You are only coming through in waves
Your lips move but I can't hear what you're saying
When I was a child I had a fever
My hands felt just like two balloons
Now I've got that feeling once again
I can't explain you would not understand
This is not how I am
[2]
Hanging on in quiet desperation is the English way
The time is gone, the song is over, thought I'd something more to say
[3]
How I wish, how I wish you were here
We're just two lost souls
Swimming in a fish bowl
Year after year
Running over the same old ground
What have we found?
The same old fears
Wish you were here
You've chosen to play apologist to a panel of critical thinkers. That's got to be unpleasant. They're only interested in evidence and sound argument, and you offer vague, unevidenced ideas, like whatever spiritual means to you.
Defiance? To your apologetics? Maybe this isn't the right activity for you. The fact that you call it defiance and that it exasperates you is what I was referring to earlier - the asymmetry between the emotional apologist and the staid demeanor of the critical thinker. I never get exasperated or emotional when posting, because why would I? Because you disagree? I don't mind that. Because I often can't get through? Why would that anger or frustrate me? I don't expect to be understood by everybody, especially when discussing philosophical and scientific matters with the faithful as we do here.
That's not my take. Religion began from the bottom with people trying to understand and control their world, and so they invented gods, which appears to be a nearly universal human activity, and tried to appease them. The comes the priesthood to exploit that proclivity. You and I have discussed this
before:
"Even today, being clergy is a great gig. No manual labor or hot sun. No education or training necessary if you want to open your own church. No government oversight. No expensive equipment needed. People bring you money every week to do nothing except tell them how to live. Instant respect and social status, although not so much as before."
This was also asked and answered in that same thread and post. Church is big business: "There's a church on every corner along with a bank, a gas station, and a fast food restaurant, and they're all there for the same reason. The marketing begins at the point of a sword with Roman legions, crusaders, and conquistadores and continues to this day with free Bibles in every hotel room and now Superbowl ads for Jesus."
How is that explained? People sometimes die for their mistaken beliefs, especially when believed fervently and zealously - like the people at Waco or the Heaven's Gate cult. Likewise with the killing and cruelty. That's to be expected in religions that define morality in terms of what they claim a god said and did.
"Religion is an insult to human dignity. With or without it, you'd have good people doing good things and evil people doing evil things. For good people to do evil things, it takes religion." - Nobelist Steven Weinberg
It sounds very human.