I've often written that I could be convinced of a religion given enough evidence. I suppose it would be fair to discuss just what that evidence would be.
Basically, anything that could truly be defined as miraculous would count as evidence toward a god. The Ebon Musings website has a page titled The Theist's Guide to Converting Atheists, which contains lists of evidence that that author would find convincing. Overall, I agree with the types of evidence listed. These include fulfilled prophecies, miraculous occurrences, direct manifestation of the divine, a scripture that contained knowledge that couldn't have been known at the time it was written, or a scripture that was entirely consistent and flawless.
However, there are many caveats. First, the evidence that the miracle occurred must be strong and convincing (as Carl Sagan said, "Extraordinary claims require extraordinary evidence.") For example, consider the case of Prahlad Jani (here's another source). He's an Indian yogi who claims that he's gone 70 years without eating or drinking. A doctor has even performed some tests supposedly confirming his claims. Unfortunately, the tests didn't use the best methodology, and the doctor wouldn't let another investigator who had experience exposing frauds be involved in the study. From the best information that outsiders have been able to gather, it appears that he's just a normal person, who was eating normally up until his time in the hospital, and then began suffering from the effects of dehydration and starvation while he was there.
As another example, consider Ram Bahadur Bamjan (here's another source on this one), who some claim is the reincarnation of the Buddha, and who others claim (since according to Buddhism, the Buddha has already achieved nirvana) is a Bodhisattva. Bamjan has supposedly sat under a tree meditating for months on end, with no food or water. However, there's no good evidence to back the claims up. There was even a screen put in front of him every night. When the Discovery Channel sent a film crew to make a documentary, they couldn't detect his heat with an infrared camera the first night they tried (makes you wonder if he'd been leaving every night all along). On a second attempt, they did observe him to go without food for 96 hours. Four days of fasting is a feat, but hardly miraculous.
So, like I said, the evidence that the miracle occurred must be strong and convincing. It can't be merely hearsay.
As another caveat, the miracle really should be something that's unexplainable by natural processes. This rules out those events that fall into the realm of unlikely, but not impossible. As I've said before, my chances of winning the lottery may be a million to one, but somebody still manages to win every week. In other words, given enough opportunities, unlikely events will inevitably happen to someone. I think one of my favorite examples of such an unlikely event is a truck that nearly went over a cliff, but ended up landing on a small ledge. You can see the pictures and read about it on Snopes. Unfortunately, vehicles fall off cliffs all the time. So, as lucky as it may have been for the driver of that pickup to land where he did, it was just the odds playing out. If, say, vehicles with Jews never went over cliffs, while vehicles with non-Jews went over regularly, then there might something to God protecting his chosen people. But in reality, religious affiliation has no effect on your chances of death by precipitous plunge.
Other, stranger seeming 'miracles', can also be explained naturally. Speaking in tongues, or glossolalia, appears very strange - speakers making vocalizations that they're convinced are other languages. However, as it's been studied, it's been learned that the speakers are using sounds that they're already familiar with from their own language (i.e. a native English speaker won't make the guttural vocalizations of German or French). Further, to quote that Wikipedia article, "where certain prominent glossolalists had visited, whole groups of glossolalists would speak in his style of speech." This certainly makes it appear that it's a learned behavior. I doubt that the speakers are intentionally lying, but I also doubt that they're doing anything more than making interesting noises.
Faith healing is an example that requires both above caveats. First, I'd need to see some strong evidence that a person was actually healed (and that the supposed event wasn't a fraud). But, there are many examples of people who think they've been healed. Many of these can be chalked up to self hypnosis or the placebo effect. A person gets caught up in the moment, and subjective symptoms are reduced. There are also a few cases where a person does get better after visiting the healer, but most of those are likely be coincidence. There's no guarantee that the cure came from the healer, as opposed to just occurring spontaneously (which does happen). Really, what would be needed is some type of study involving a control group, to see just how effective faith healing really is (though I doubt many researchers would be willing to deny patients real treatment for such a study).
Another consideration is that many claims for the divine cross religious borders. If a Christian and a Hindu have both claimed to have felt a god's presence, or to have received a divine message, which one of them should we trust more? Should the Christian or Hindu evidence be counted equally? Doesn't it seem more likely that it's really all in their heads? If supposed miracles occur just as often for diverse people regardless of their religion, then it doesn't argue strongly for their particular religions. This does leave open the possibility of a non-denominational god, but it seems more likely that there are more earthly causes that are being misinterpreted.
Prophecy requires special mention. Of course, these require the same standard of evidence as other miracles. Pointing to a prophecy in an old book, and then pointing to another section of the same book that says the prophecy was fulfilled, isn't very convincing without independent sources confirming the claims. But for prophecies to be convincing, they must also be specific. Nostradamus's writings are an example of how bad this can be. When every generation can interpret a prophecy to be applicable to events happening in their own time, then the prophecy probably isn't specific enough. Prophets must also have a decent accuracy. As the saying goes, even a broken clock is right twice a day. If someone throws out enough prophecies, some are bound to come true. Prophecies must also be unlikely, or something that would be very difficult to predict otherwise. It's not very profound to prophesize that the next round of presidential candidates will lie during their campaigning.
Ideally, miracles should be witnessed by more than one person. Personal revelations fall into this category, especially considering, like I wrote above, that people from diverse religions are all convinced that they've personally felt their god's presence. Another example is the case of Adele Brise. She claims to have seen and spoke to an apparition of the Virgin Mary. The thing is, there were two other women with her, neither of whom could see Adele's vision. Despite the unverifiable nature of the event, the Catholic Church has officially decreed that "these apparitions as worthy of belief". There are many reasons a person could claim to see apparitions or hear voices. The simplest, though I like to hope the least common because I like to think the best of people, is that they're simply lying - looking for attention, a way to make a buck, or even just playing a joke. But there are also people who really do have mental problems, who have hallucinations or hear voices in their heads. Just because some of them claim the voices are coming from on high doesn't mean that we shouldn't give them the appropriate treatments they need.
Assuming that a miracle did meet the appropriate standards, I can say that I still wouldn't be convinced by a single example. There are too many other possible mechanisms. For example, let's just say that a prophet came along who truly could predict the future. It's possible their insights were divine, but it's also possible that seeing into the future was an ability of humans, and that most people simply aren't very good at. I would want to see research done into where the prophet's ability came from. I wouldn't immediately jump to accepting their religion.
There are other possibilities that may seem outlandish, but no more so than Thor being a real deity. As Arthur C. Clarke wrote, "Any sufficiently advanced technology is indistinguishable from magic." It's a big universe out there, and it's possible we're not alone. SciFi shows like Star Trek make us want to think that advanced civilizations would be noble and peaceful, but we don't know that they would. Maybe a civilization wanted to conquer our planet, but their space ship didn't have the resources to do it outright. A few magic tricks to impress the natives would go a long way. Or maybe aliens would be practical jokers, having a bit of fun at the primitive apes' expense.
An important consideration here is the timing of miracles. As far as I've seen, there haven't been any well documented miracles. But even the claims of miracles show a decrease in grandeur over time. The Bible describes Moses parting the Red Sea, while a more modern miracle I described above was a woman seeing a vision her companions couldn't. It really seems as if many supposed ancient miracles are myths, and many more are due to people not understanding how the universe really works. As we learn more and more about the universe around us, and as documentation of events becomes better and better, miracles become more and more minor. But if there truly were a god, it wouldn't have to be that way. A god could continue performing major miracles throughout the ages. Considering the dearth of miracles in the past, the source of any new 'miracles' would have to be carefully considered.
My discussion of miracles above wasn't exhaustive. There are other classes of miracles that I didn't discuss. But I think that this gives a sense for the standards I'm looking for. Miracles must be well documented, not explainable by natural means, and more than simply unlikely events. And even if a supposed miracle was well documented, we must consider other possibilities before accepting it as evidence for any particular religion.
My parents bought me an interesting present for Christmas a couple years ago. It was an old book published in 1922, titled God- or Gorilla? How the Monkey Theory of Evolution Exposes Its Own Methods, Refutes Its Own Principles, Denies Its Own Inferences, Disproves Its own Case. As you can guess, the author, Alfred W. McCann, was not a big fan of universal common descent. (For those of you who may be wondering, my parents aren't creationists - they bought the book because they knew I'd find it interesting.)
I wrestled back in middle school and high school. I was never great, but I wrestled varsity many of those years and won more matches than I lost*. My senior year, two girls decided to join the team. This was the first time it had ever happened at my high school. Before the season started, the rest of the team talked about it a bit. We wondered if it was appropriate to wrestle a girl, and whether or not they should be on the team. Well, as soon as practices started up, it just kind of fell into place. They were just two more wrestlers, and were treated just about like everybody else on the team (with the exception of using a different locker room and getting weighed in separately). It turned out to not be a big deal at all. One of the girls was pretty good for her first year. Obviously, they were wrestling JV, but this girl won a lot of her matches, and even pinned a couple of her opponents. This was all about 15 years ago.
I've gotten permission from 