The late Carl Sagan once declared that he has a fire-breathing dragon living in his garage:
"Show me," you say. I lead you to my garage. You look inside and see a ladder, empty paint cans, an old tricycle -- but no dragon.
"Where's the dragon?" you ask.
"Oh, she's right here," I reply, waving vaguely. "I neglected to mention that she's an invisible dragon."
And for every test that the skeptic proposes, Sagan had an excuse for why the test won't detect the dragon. It floats in the air; it's incorporeal; the flame it blows is heatless; and so on.
Naturally Sagan didn't actually believe he has a dragon in his garage, but he was making a point about religion and the invisible, incorporeal god that many people believe is in their garage. Instead of gathering evidence to support the idea of the dragon in the garage, believers insist that we accept the existence of such an invisible, soundless, heatless, incorporeal, undetectable dragon unless it is disproved. But of course it cannot be disproved, because there's an excuse for every failure.
Under normal circumstances, we treat the failure to find expected evidence as almost as good as positive evidence. In a murder trial, the failure to find gunpowder residue on the accused shooter can legitimately cast doubt on the claim he was the shooter. But such negative evidence is only useful when there is a clear-cut pass or fail. You can't accuse somebody of shooting the victim, and then when no evidence supports your accusation, turn around and say that the murdered must have used a special gunpowderless gun that fired invisible bullets that left no visible wounds.
God is invisible, that's why you can't see him.
In the face of such special pleading, then the failure to disprove the claim doesn't mean anything. There is no way to disprove the existence of god, because for every test there's always an excuse after the fact why it didn't work.
The game is always rigged, and you will lose if you play.
The only thing you've really learned from my insistence that there's a dragon in my garage is that something funny is going on inside my head. You'd wonder, if no physical tests apply, what convinced me. The possibility that it was a dream or a hallucination would certainly enter your mind. But then, why am I taking it so seriously?
(Thanks to PhillyChief.)
Believers will counter that of course they have positive evidence for their god. (It's evidently only other gods that are illusionary or mythical.) But the problem with the evidence given is that it either has other explanations ("see, there are no elephants living in my garage, because the dragon ate them"), or that it's entirely subjective. Your epiphany is my bad burrito -- and contrariwise, the awe and sense of wonder I have when I contemplate dirt is a never-ending source of amusement for Mrs Impala and her friends. (Some people hug trees. I play with dirt. If rocks are the bones of the Earth, then dirt, earth, is the flesh. Carl Sagan famously said we are all star-stuff, but the star-stuff had to become dirt before it became us.)
I can put my hands in the dirt, I can touch it and weigh it and dig it over, and if I treat it right, it will bring forth all manner of life. Perhaps that's the difference between religion and spirituality: spirituality is about subjective feelings related to real things, while religion is about subjective feelings about imaginary things.