The OP is somewhat true, but it's not something unique to religious language. I doubt I'm the only one who has been asked, "What is that?" about some (non-religious) word and found myself struggling to give an adequate definition. The reason (as has been suggested in this forum several times), is that most language is learned by means other than sitting before a scribe handing out definitions. Most language is learned by experience and context - something very difficult to reproduce when asked for a definition.
I don't know about that....
It seems to me that we have a LOT of words that describe things that none of us has (or even
could) directly experience, yet we all know exactly what is being talked about.
A simple example: "atom".
Sure, we understand what "matter" is, as we are made of it and touch matter all the time, but "atom"?
Not really. "Electron" even less. "Photon" is something that
common sense would dictate is completely ridiculous.
Yet all of us, even the most die hard creationists, know what is meant by that. Even if most of don't really understand the physics involved.
See, I think what the OP is getting it, as that for such words to have any meaning, they should actually reflect something
in the real world.
Eventhough we can't "see", feel, touch, etc a photon - we all have a rough idea of what is being talked about. We all understand that the word is defined by an actual real particle in reality. Eventhough most of us, again, don't really understand the physics involved.
The problem with the ambiguous,
abstract and borderline-poetic words used by theology and the religious in general, is that it is not clear at all, how the words actually relate to reality.
When people say "god is love"... it doesn't mean much. Because we understand that love is an emotion and that emotion is, ultimatly, brain chemistry. And when you ask a theist if they mean that "god is brain chemistry", off course they say "no".
When people say "god is energy"... it also doesn't mean much. Because we have physical definitions of what energy really is... And when you ask a theist "is that what your god is? radiation and heat etc?", then they say "no". And again we are left wondering what is being meant.