What is truth?
Pontius Pilate asks this question (John 18:38), as have philosophers and intellectuals since the beginning of recorded history. And no one has been able to supply a satisfactory answer. To be honest, I don't know what's true; I don't even know what truth is. And neither do you. Nobody knows.
But I do know what falsity is. If you tell me that the sun will rise tomorrow over here, and it rises tomorrow over there, you're mistaken. I'm absolutely certain you're mistaken. I don't necessarily know where you're mistaken, but I'm certain that you're mistaken somewhere.
I can't just say that the opposite of mistaken is correct, because the law of the excluded middle doesn't always work in real life: there's mistaken, correct, and unsure. Even if you're not mistaken, you could be correct by accident or coincidence, or you could be fooling me, or you could be correct in a limited sense, only by virtue of making some approximation, but mistaken in the larger sense (as was Newton regarding gravitation).
This observation about how I have to construct my ideas about the world stands in stark contrast to canonical logic. As Gödel showed us, we can prove that we are correct that 2+2=4, and we can prove that we can prove ourselves correct (and prove that we can prove that we can prove... etc.), but we can't ever prove that some mistake really is a mistake: We can't prove that 2+2=5 is a mistake.
For this reason, I'm not just suspicious of, I'm positively dismissive of any attempt to prove anything about the real world using just logic. Logic is a terrific — indeed indispensable — tool, but it is just a tool. It doesn't get to the bottom of things at all, and we know it cannot. It works extremely well for speaking precisely, but it doesn't ensure we are speaking truthfully, indeed it cannot tell us at all if we are mistaken.
We didn't make very much progress in knowing things about the world until we stopped being hung up on knowing and accepting the truth and started being very careful about knowing and rejecting falsity. Genius is not about how quickly or accurately you can perform complicated intellectual tasks. Genius consists of looking at something that everybody knows, realizing that it's mistaken, and explaining to everyone else precisely why it's mistaken.
And the genius of those who developed The Scientific Method, from Galileo to Popper, is realizing that what everybody knew — that the search for knowledge was the search for truth — was mistaken! The search for knowledge is the search for falsity. Sherlock Holmes hit close to the mark when he said, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth." Whatever remains might not be the "truth" — whatever that is — but that's what you run with. If tomorrow you find it's mistaken, you throw it out and you keep working.
Philosophers are, understandably, uncomfortable with this mode of reasoning. Too bad for philosophers. The scientific method is at least "self-referentially coherent": If the scientific method were mistaken, we would know it; that we have not yet found it mistaken, even after very thorough, hostile examination, is sufficient warrant to run with it. If we find out tomorrow it's mistaken, we'll look for something else; we'll cross that bridge when and if we come to it. For now, we've never proven the scientific method mistaken: Every mistake ever made by every scientist has been attributable to either not having enough evidence, not examining enough possible explanations, or using some method other than the scientific method.
And that's what bugs me about theists. I've never had any theist explain — and commit to — a method by which I might in principle discover that she is mistaken. Not one. Ever. I've seen a few theists float trial balloons, but careful examination has shown me that when I apply the method they propose, they are actually mistaken. And when I point this determination out, they withdraw the method. It doesn't have to be the scientific method; the scientific method is not assumed a priori. We don't have to use perceptual evidence that everyone (or most everyone) affirms. I personally am stumped for an alternative, but hey, I don't know that I'm correct, I know only that I'm not yet mistaken.
This is the problem with faith, theism, woo-woo and conspiracy theories in general. They may well be "true", whatever that means. But until I have some way of determining in principle that some idea might be mistaken, I just don't know how to think about it. I don't know how to find a way to believe it.
I have no method to determine in principle whether or not Christian theism is actually mistaken. The scientific method won't work: Christianity is compatible with everything I might observe, whether I actually observe it or not. By the same token, I have no method to determine whether Islam, or Hinduism, or Buddhism, or Norse theism (Odin, etc.) or any other woo-woo bullshit might be mistaken. These might all be better or worse literary metaphors, but they have nothing at all to do with truth and — more importantly — falsity.
This situation is very different, for instance, for evolution, or relativity, or quantum mechanics, or thermodynamics, or any other scientific theory: I do have a method, the application of method would detect a mistake, people have applied the method and found that these theories are not yet mistaken.
Skeptics, careful skeptics, look for falsifiability. We — like everyone else — have no clue what "truth" is, so we concentrate on detecting falsity. If we can detect falsity, and we don't actually detect it, well then, we have some basis for confidence. But if we cannot in principle detect falsity, we have no basis for confidence. Skeptics also have to bend over backwards to be honest: to have confidence in some idea, we have to assure ourselves that we have looked as hard as we can to find falsity, that we have considered every possibility and looked at all the evidence.
Furthermore, because we know we cannot actually evaluate every possibility, because we cannot examine all the evidence, we can never be absolutely certain that our ideas are not actually mistaken. All we can say is that our ideas are not yet mistaken, and our confidence in our ideas is warranted precisely to the extent that we have undertaken a thorough and honest search for mistake. And to have confidence by this warrant requires that it means something to discover a mistake.