This question is usually posed to me after an ongoing discussion has come to a complete standstill and both parties are ready to call it a day. That's when someone will ask me, "What evidence would convince you in a God?" This is the question I get after spending countless hours interacting with someone about the problems with subjective testimony and the need for verifiable objective evidence.
What is the problem with subjective testimony? Ordinarily, there isn't a problem with subjective testimony. We use it in our everyday lives when we ask someone how their weekend was. After telling us of a camping trip or some other activity we rarely look them in the eye and respond with, "I don't know, Bob, I'm going to have to see some verifiable evidence. You may have been at home all weekend playing XBox." Most circumstances don't give us cause to question the testimony.
The problem comes into play when there is a conflict between objective data and the subjective testimony. The testimony of the Bible is in direct conflict with everything we know about physics. We have miracles, talking animals, and all sorts of various magical claims. When a skeptic questions how this stuff could have occurred the only reply we seem to get is, "God can do anything." God can do anything, so therefore logic is out the window? Do we not question an account simply because it's been canonized as religious doctrine?
My first thought that is that such a story is a fabrication. But, let's put that aside for now. Let's assume that everyone that contributed to the Bible believed 100% what they wrote. Does that mean that we should then accept the testimony as true in spite of the conflict with our understanding of the physical world? When there's a conflict between subjective, non-verifiable evidence and reproducible, objective data, I'm going to have to side with the objective data. It's the one thing that everyone can test for themselves. Is the data infallible? No. Absolutely not. Our interpretation of the data maybe incorrect or the data itself maybe invalid for one reason or another. But, unless an error can be found with the objective evidence, than it stands to reason that the objective evidence has higher standing than the subjective testimony.
So, how could all these people that believe they've had a supernatural experience be wrong? They must have experienced something? True. Let's say that they themselves were fooled or deceived by someone else. That's not an unreasonable proposition. When it comes to Biblical testimony we don't have any way to verify the human authorship. Certainly, with some books of the Bible, there's strong evidence to suggest that the writings were dictations or collections of word of mouth stories, more so for the Old Testament than the New Testament. But, even the New Testament books sometimes will explicitly state that they were dictated. So, in a lot of instances, the author, whoever that may be, is writing from a degree of separation from the actual events. They might fully believe the story they are writing down, but that doesn't mean it actually happened.
Beyond that, the brain is a very untrustworthy device. In altered states, whether it be drug induced, dehydration, or mental disorders, the brain can be fooled into seeing things that aren't there. A person can have entire experiences that never happened, but the brain produced something that can be considered to be a "waking dream". Some of the experiences being told as a religious experience may just be a malfunctioning brain.
Even fully functioning brains aren't to be trusted. Of course, there's confirmation bias to consider. There's also false memories, where a past event is remembered differently than how the event happened. How many "recovered memory" accounts of childhood abuse has turned out to be false memories that were created by someone that never did actually experience anything of the sort? There's many documented cases of the problems of "recovered memory" accounts.
What I'm driving at is that there's many reasons to doubt a subjective testimony even if the person themselves giving the account is fully convinced that the events occurred. When there is a conflict between the subjective experience and the objective data, I don't see how it's rational to discard the objective evidence and side with the less reliable subjective testimony.
So, what evidence would I need to convince me there was a God. The answer has always been that I would need objective, verifiable evidence. I have been posed with the question, "What if you, yourself had an interaction with God? Would that convince you?" The truth is it might. But, if that did actually happen, would I have any expectation for others to accept my testimony when I wouldn't have any objective evidence that could verify that my account is even physically possible? No, of course, not.