Have you ever had a dream? Describe to me that dream? Prove to me the contents of that dream? It's impossible. You can't prove to me what that dream was about, it was only in your head. But you KNOW that you had that dream, and it's content. I can call you a liar and a fraud, but you know, and have faith, that that dream was real. This is the same way I believe in God. If I know God exists, for the same reason you know your dream exists, can you say that my God and your dream both don't exist?
I've seen this question posed in a number of ways on different websites over the years, but I've never seen a decent answer for it. For the record I'm an atheist (agnostic atheist) but I just happen to love trying to see both sides and I've personally not seen a good (in my opinion) answer for it. It hits a nerve with me because I dream regularly, and if I was asked to prove the content of a dream I couldn't. It's only a truth to me personally.
The best answer I can think of has to do with content, that a dream is a personal relaity but the dream doesn't mean anything to someone else, where as the concept of a God, by definition, is to be agreed upon by many. But in a strict theist sense, there's no reason why a god couldn't reveal themselves to people on a personal level, and provide no evidence.
Thoughts, challenges, answers?