I agree, and that is on those proposing (or asserting) the existence of a god or gods. That is also why "I don't know" remains the only valid answer.
People can propose anything they want. And they can believe anything they want about it. And they can assert that you should believe it as well. But at that point you have the right to ask them to explain why you should agree with or believe their assertions.
It's actually fairly rare that anyone will assert that we should accept and believe as they do. Yet a lot of people are way too quick to jump the gun on demanding justification, anyway. Usually because they have every intention of rejecting any justification they might receive, or rejecting the proposal all together because they didn't get exactly the kind and amount of justification they demanded.
That's a little disingenuous. We're obviously talking about the existence of some kind of real being.
That's not obvious at all. And anyway, what the heck is a "real being"? By my understanding theism (the philosophical proposition that God/gods exist in some manner that effect humanity) is based on the logical necessity of some unknown/unknowable source enabling the possibility of existence. It does not define that source, nor does it even propose that we could. It is presumed that this mystery source must logically be transcendent of the existence that it enables, and is likely to be "intelligent", but the proposition itself does not require nor address either of these issues.
Literally anything can exist as an idea, but the effects of ideas are entirely distinct from the effects of something real.
Ideas are as real as reality itself. In fact, "reality" IS an idea. Without the idea of reality, NOTHING is real. I think you're trying to claim that objectivity defines reality. And no philosopher anywhere is going to let that stand.
I'm not denying the potential significance, and even positive benefits, that come from various theistic beliefs.
I think, here, you are confusing religious beliefs with theism. These are not the same area of humans thought. It is true that religious conceptions of "God" can be very helpful to people. (Or unhelpful.) But religion is a result of theism, it does not define theism.
I am just saying that they are largely unrelated to the questions of whether anything in those beliefs are actually true in any way (indeed, the influence is often maintained by avoiding those difficult questions).
What is true is that those god-conceptions work for the believers in a useful and positive way. It's why they believe. What is also true is that the mysterious-yet-always possible nature of the 'God' idea make it amazingly versatile and yet still effective for a great many people. And what is also true is that there is no way to determine that 'God' is not possible, or even not probable.
The latter is the focus of the OP and the basis of the "I don't know" starting point. The former doesn't change any of that, though it could certainly be presented as part of an answer to the question of where a person can go from that starting point.
"I don't know" may ultimately be the only answer we humans can honestly give to any question asked. As we can never be truly certain. But once given, we have nowhere else to go. Nothing comes after "I don't know". And therefor no benefit comes if it, either. That's why for me, "I don't know" seems more like it ought to be a starting point, rather than the end point. "I don't know, but ... (I think, or I feel, or I surmise, or I presume, or whatever)". And then see where it takes us.
