The theists in the room were trying to play a word game within a word game. Get the reader to admit that the prefix a- is added after the root already exists. (I defeated this by showing in Plato's Apology that even the game fails, the word "atheist" in its earliest use, was against the guy who denied sun and moon worship). The scam is fairly innocuous, so the reader tends to miss it and make an implicit admission that spirituality came first.
So the game goes like that, you plant this idea in the mind of the reader, and they forget it was a fallacy, they implicitly agree to it, then they go about arguing for or against religion, and all the while the theists are quietly gloating that they pulled one over on the atheists.
Classic BS. The OP was not so disingenuous. It was posed honestly enough. Post #2 is where the deception, the word game, comes into play. Had it not been for that, I would have dropped my ideas at the doorstep and moved on.
But there is something insidious about pro-religious deception. It's almost pathological - the wolf in sheep's clothing. So I took an interest and followed the dialogue, throwing in my 2¢ worth wherever the fallacy cropped up and grated at me.
Your comments bring this back to the notion of when did religion take root in our developmental history. As you say, it's a matter of common sense. I agree with you.