There's a difference between being able to perceive something and being able to describe it to others. Languages have their limits, and evolve to communicate ordinary, everyday situations. Add the fact that the mystical experience posits a heightened or expanded consciousness, ineffability would be expected.
I've done plenty of psychedelics. These are famous for putting the user in a similar state of mind as those experiencing revelation or being enlightened. Yes, I agree that the subjective experience defies verbal description. But we don't have to be able to describe the subjective experience. We can stop at describing what is happening in the brain. Even though we have a very limited understanding of the brain, I think we have the scientific knowledge necessary to pull the blanket off of any religious mystical experiences. It can all be explained by pretty mundane scientific explanations. You don't need to be particularly well read on science to realise that our systems of perceptions aren't built for accuracy. They're built for speed. They rely on a bunch of inbuilt assumptions to speed up the process. While most often correct they can fail in spectacular ways.
To quote Neil DeGrasse Tyson:
"If we were honest about shortcomings of human physiology then "optical illusions” would instead be labelled “brain failures”"
I think we know enough about the brain to rule out magic as a contending explanation for mystical experiences.
The argument goes like this: In order for us to be able to evaluate whether to believe in something or not, we have to be able to define it. If only to talk about it intelligently. If we can't, then I just shift it over into the category of things I cannot possibly know and I stop worrying about it.
So if something can't be defined its reality is doubtful?
That's not what I said. Both parts are wrong.
1) Some things can be defined just fine, but we still can't explain it accurately. One example is the weather. There's no mystery regarding the forces of nature create weather. Beyond just a few days the maths just gets too complicated. There's nobody who can do it. We know we should be able to work it out. We just don't know how. Until then we'll just have to accept that there's a large degree of uncertainty regarding the weather during our trip to Spain this summer. That's what I'm talking about.
2) If something can't be defined in a meaningful way then what does it mean that it "really exists"? Have we in fact said a true statement (or false)? It equivocates between non-definition and non-existence. Non-definition just means we can't talk about it intelligently. It doesn't mean I deny it's existence or possible existence. As regards to the OP. I think it's valid to question the point of discussing God until it is well defined. What's the point in believing in something that may hypothetically be true? Isn't that the same thing as believing in anything? Literally anything. Even utter nonsense.
I wonder how an uncontacted, Amazon native would describe his experience to the family, were he suddenly transported to the middle of a modern city for a day? Even without neurological alteration this would be a challenge.
I don't know. But we do know how people historically have described things they don't quite understand. Via metaphor of stuff that they do know and understand. And then hope that the analog is good enough. Here's a fitting quote:
"Because we do not understand the brain very well we are constantly tempted to use the latest technology as a model for trying to understand it. In my childhood we were always assured that the brain was a telephone switchboard. ('What else could it be?') I was amused to see that Sherrington, the great British neuroscientist, thought that the brain worked like a telegraph system. Freud often compared the brain to hydraulic and electro-magnetic systems. Leibniz compared it to a mill, and I am told some of the ancient Greeks thought the brain functions like a catapult. At present, obviously, the metaphor is the digital computer."
-John R. Searle, MINDS, BRAINS AND SCIENCE, p 44
So to answer your question. I think the Amazon native would describe it wrongly and we would be amused if we heard it.