Recently, Greta Christina wrote a piece on AlterNet about what evidence would convince her that she is wrong: God (or in one case, a supernatural soul) actually does exists. If atheists claim to value scientific reasoning, than our beliefs should be demonstrably falsifiable. By pointing out reasonable ways for our belief, that god does not exist, to be shown false (or that we can be convinced it is probably false), we are showing others that we are as open-minded and reasonable as we can be. You can't reasonably ask for much more than that from us. So I thought I should present my own list of things that would convince me that I was wrong about God.
I should start by saying that I mostly agree with Greta's examples. If there was a clear, unambiguous message from God that a significant number of people in many locations received, it would cause me to at least consider that the existence of a god is much more likely than I currently believe. Sam goes for evidence showing that a particular religion has vast benefits than are best explained by supernatural forces. For example, prayer tends to be much more effective for this religion than all others, followers of a specific religion tend to be much healthier, or wealthier (or wiser :-)).
Of course, this type of evidence is still mitigated by the well-known problems with explaining why the world is the way it is, if a god actually does exist. Why does evil exist? Why do seemingly good people suffer? To me, there are two possible solutions to this problem. First, the examples presented above could be so convincing and unambiguous that I would have to conclude that god exists, enough to conclude that my objections above are more easily explained than the evidence presented. On the other hand, someone (perhaps God, or just a smart person) could also give me a good answer to the problems of evil and suffering; a logical argument or explanation that I had never come up with by myself. In that case, the standard for evidence could be a bit lower, because I would no longer have to contend with the perplexing problems of evil and suffering.
I tend to be a logical guy, so if someone could present me with some arguments that deal with my common objections, I suspect the evidence needed to convince me God probably exists wouldn't be too extraordinary. It would still need to be of the same character (a clear message, effective prayer, etc.), but it wouldn't need to mitigate the fact that a so-called loving god seems to be a giant dick.
An objection I anticipate is that these examples are not reasonable. It's true that some examples sound extreme. However, we have to remember that this is evidence for a supposedly omnipotent, omniscient, supernatural being. In fact, any evidence for such a being has to be extraordinary, enough so that the supernatural explanation (such as a god) is more likely than a natural one. If the evidence isn't extraordinary, why would it require a god?
Finally, I suspect that if a god did exist, there are a number of ways he could convince me that I can't conceive of now. I'm tempted to bring up the argument that if God is omnipotent and omniscient, then he would know what would convince me, and have the power to do it. So if he exists, and he wants me to believe in him, I should already believe in him. While it does seem like a convincing argument, it also seems a bit like begging the question: if I should believe in God, then I would already believe in him. Perhaps it's not the perfect argument, but it is at least a piece of evidence that either God doesn't exist, or he doesn't want me to know he exists for some reason (and no, the argument that God wants me to believe based on faith is not convincing).
So you the faithful (or perhaps God himself, if I'm important enough to garner his attention) appear to have two avenues to change my mind. Either present some extraordinary evidence, or convince me that common problems (such as the problem of evil or suffering) are not really problematic, and then provide a much more reasonable set of evidence.
What about all of you? What would convince you to change your mind?