As I've said many times before, faith is not a valid way of determining the truth. It's impossible to separate faith from wishful-thinking, and with faith, there's no way to tell when you're wrong.
Evidence-based thinking, as in the scientific method, is different. It's still easy enough to fool ourselves, of course, but science gets around that with a community of like-minded individuals all eager to point out each other's mistakes, if not their own. The ultimate result is a consensus that's far likelier to be the truth than with any other method yet discovered.
This is why we have a worldwide consensus on scientific issues (not cutting-edge science, of course, where the evidence is still being sought), while religious belief varies depending on where you were born and what you were taught as a child. With faith, you can't tell when you're wrong, and you certainly can't demonstrate to others that they are wrong, so everyone just ends up believing whatever they want to believe.
The scientific method works superbly for scientific issues, and there's much we can learn from it for ordinary questions as well - mainly that we should require evidence for our beliefs, that we should always remember that we could be wrong, and that we should be willing to change our minds if new evidence demonstrates that. Of course, this requires that we listen to contrary arguments, even after we've made up our mind. (We don't, however, have to listen to the same contrary argument, over and over again.)
So, OK, theoretically I'd change my mind if you show me sufficient evidence. My disbelief, after all, is based on a lack of evidence. It's not that I believe there is no god, but that I don't believe there is a god. See the difference? The Flying Spaghetti Monster could exist - I can't disprove it - but there's no good evidence to indicate that He does. If you want me to believe, show me the evidence. So what kind of evidence would I need? Well, that's difficult to say.
For one thing, what kind of god are you talking about? Inevitably, whenever atheists discuss religion, believers complain that we're not talking about their god. I'm not some bible-thumping hick who thinks the world was created in six days. My god is different. My religion is sophisticated. Since there's no consensus in religion, it's hard to pin down just what a "god" is supposed to be. And it's hard to speak in generalities when you're talking about evidence.
It's particularly hard to get a grip on a deist's god. Maybe you think some vaguely-imagined "god" ignited the Big Bang and started the universe rolling, but hasn't cared about anything since then. There's no heaven and no hell. God doesn't care what you eat or who you sleep with, and doesn't even know you exist.
Maybe that's "sophisticated," I don't know. It's certainly easier to get along with that kind of believer. But I can't imagine what evidence could possibly convince me that such a god exists. If your god has had nothing to do with this universe for billions and billions of years - and probably was never in this universe to begin with - what kind of fingerprints could he have left behind? Clearly, you don't believe this because of the evidence. (I have a hard time understanding how - and why - you believe it at all. But people do.)
Of course, relatively few people seem to believe in this kind of god, but you see the problem. Now I can disbelieve in all gods for the same reason - because of the lack of evidence. But to specify what kind of evidence I'd require to change my mind, I need to know what your god is supposed to be and do (or have done). Science generally takes a hypothesis and makes predictions based on it. The hypothesis must be clearly defined, so you know what result will confirm it - and which will disprove it.
So what evidence would clearly confirm the god hypothesis? If a being created our universe, would such a being automatically be a "god"? What if the Big Bang was initiated by alien scientists in some other universe, playing around with their equivalent of the Large Hadron Collider? Would you consider those aliens to be gods? We ourselves can do many things that would have seemed god-like to our ancestors. Are we gods then? Most people would say no.
At this website, an atheist lists what would convince him. These things would be pretty good evidence - and the lack of them is pretty good evidence that religions aren't true - but would they really be incontrovertible? Any sufficiently advanced alien species, or even time-traveling humans from our future, could probably manage them, and either would be a simpler explanation than an omnipotent, omniscient "god."
Still, as I say, these things would be evidence, even if they didn't completely settle the question. Believing in a god would be far more reasonable if such evidence existed. Would they convince me? Well, I don't know. The problem is, I can't think of any evidence for a god that would conclusively preclude other explanations. And almost any other explanation would be simpler. Occam's razor, you know.
That doesn't mean the simpler explanations would necessarily be correct, but just that we shouldn't assume otherwise. And really, this is the whole point of looking for evidence, isn't it? On the other hand, no theory is constructed - and certainly not convincingly demonstrated - from the first bit of evidence that comes to hand. If this kind of evidence did exist (and note that it doesn't), it could start us on the right path, I suppose. I don't know how we'd rule out other explanations, but scientists are clever at things like that.
But I'm not clever enough to come up with potential evidence which would immediately convince me to believe in some generic "god." In theory, yes, evidence is all it would take, since it's the lack of evidence - good evidence - that makes me an atheist. But as a practical matter,... well, how do you conclusively identify a "god," such that other knowledgeable, powerful entities are excluded? You might claim that gods are supernatural, but that just begs the question.
Actually, I think we're looking at this the wrong way. I'm an atheist because I see no good evidence for a god of any kind. But to change my mind, you only have to convince me that a specific god exists. It just takes one. Although there are all sorts of beliefs in the world, all of them don't have to be correct. Indeed, they tend to be mutually contradictory. And if you're a believer, you don't care about all those other gods, anyway. Your belief is specific. So why should we be talking in generalities at all?
In most cases (other than deist belief, which I discussed earlier), your God is supposed to do and have done specific things - right here on Earth. Your dogma includes specific claims. And since it's far easier to look at specifics than generalities, we should expect an easier time finding evidence, both for and against, these specific claims.
Do you think that God causes a statue to weep blood? Well, it's easy enough to analyze the blood, to examine the statue, to restrict access and see if the flow continues. And in every case so far, where this was permitted by church authorities, it's been found to be a fake. That doesn't prove that no god exists, or even that your God doesn't exist. But it does show that this specific claim is erroneous.
Not all claims have to be that specific, either. I hear believers claim (without evidence) that God must have guided evolution to produce human beings. However, there is not just abundant scientific evidence for evolution, there's plenty of evidence that disproves any sort of intelligent designer. Well, not any sort. An omnipotent, omniscient god who actively tries to trick human beings might do this, perhaps because he enjoys burning souls in Hell. But this isn't your God, right?
This is where being specific really helps. It's hard to find evidence that would incontrovertibly apply to any generic "god," but believers don't care about generic gods anyway. So why bother? You care about your specific God and your specific dogma. Do you have evidence to back them up? Are you willing to challenge your beliefs by looking for evidence, both for and against?
I'm an atheist - I've always been an atheist - because I see no evidence that a god or gods exist. Good evidence of just one god, any god at all, would change my mind. I don't know what that evidence would be, but then, I don't know how you distinguish a generic god from some other being with godlike powers. But you don't have to convince me to believe in a generic god. Any god will do.
If you have reason to believe in your God - and presumably a reason that demonstrates why all those other religions are mistaken - I'd love to hear it. But I'd start small, if I were you. Do you have evidence to back up any of your religious beliefs? And I don't just mean that you feel they're true. "Believing in your heart" is no different from wishful-thinking. "Evidence" means something that could potentially demonstrate that you're mistaken.
* Note that this question has been debated recently at other websites. PZ Myers addressed it, and so did Jerry Coyne and Greta Christina, among others. I haven't read any of these, for pretty much the same reason that I don't read book reviews before posting my own. I wanted to think about the question myself, without being overly influenced by other arguments.
There's a problem with this, of course. I could easily miss an important point. I could, potentially, change my mind after reading their arguments (I do know that they disagreed). After all, my opinions have been molded by what I've read, and by what I've heard, from others. My whole post could seem foolish, if I don't read what very capable people said first (this is always my concern with book reviews).
Often enough, my posts are comments on what other people have said. But in this particular series - and in my book reviews - I try to think questions out for myself. For good or bad, these are my own opinions. Maybe I'll regret omissions, maybe even my general conclusion, once I find out what other people are saying. But there's no point to this kind of post if I just regurgitate someone else's views.
Well, that's what I'm thinking right now, anyway. :)