I saw this question posed recently on a Christian blog and I have been thinking about it for a little while and figured I would make a post of my own. In the post on the other blog there was a quote from Dawkins and another one from Sam Harris (I believe) each to the effect of "No evidence could convince me that a God exists". I have been trying to decide what I think about this and I have actually gone back and forth a little bit, at any rate it seems like an interesting discussion topic and I figured I'd make my own post about it. (For anyone wondering, I was going to post in the comments of the blog in question but it devolved into name calling pretty quickly and I didn't feel like jumping in to it)
My first reaction to reading this was to wonder whether the quotes were taken out of context. It seems strange to me that Dawkins or Harris would say something so definitive. I think such a thing is incredibly unlikely, I would be very shocked to find out one of them decided they believe in God, but even still it seems odd that they would make such a strong statement. I thought that perhaps they were speaking colloquially or something. I tried to honestly answer the question of what evidence I would accept as the proof of God's existence for a while and the more I thought about it, the more I think these were probably fair quotes of theirs.
Let me explain with an example. Suppose I was an amputee and Jesus showed up to my apartment one day and regrew my arm. That would be awesome! I would be extremely grateful to him. After having this experience, what can I conclude? This guy that claims to be Jesus is incredibly powerful. He either has magic or technology that is beyond my understanding. Does it mean that the guy in front of me is God? Does it mean that he has the power to do anything I can imagine? Does it mean that this being created the universe? Does it mean that when I die I will either go to eternal bliss in heaven or eternal torture in hell? I don't see how I could answer yes to any of these questions. All I can say for sure in this situation is this guy who claims to be Jesus is extremely powerful and beyond my understanding. Maybe Christianity is true and Jesus stopped by to help me out, it is certainly a possibility here. Maybe there are aliens who know how to regrow human limbs that are disguised as my conception of Jesus. How can I be sure? How can you ever make the jump from any particular evidence to all the wonder that is God? My guess is that this kind of thing is what Dawkins and Harris were getting at.
So to return to the original question, what would it take to convince me that God exists? I don't know, nothing I can imagine would do the job, but if God really does exist, he is all knowing, so he should know what it would take, and he is all powerful, so he should be able to do it.