Here's my side of the story. I expect to be ripped apart and downvoted quicker than I can say "Eep!". This is how God proved His existence to me. I do not make any claim that this automatically becomes a concrete proof for you. Please read all of this before exercising your voting rights!
I've been a Christian for several years, experiencing a gradual conversion. No road to Damascus dramatics, He didn't need that. It just became more and more apparent to me that God exists, and that I need Him to guide me and eventually to save me into His kingdom.
I learnt about the amazing sacrifice of Jesus; I learnt of the nature of God, life, heaven and hell, what the Bible teaches about pretty much everything. I even heard God speak to me on a couple of occasions: once audibly, once via a bizarre German camper van with a Bible verse on the back.
None of this counted as proof, though. I believed, but I wanted to know. Call it lack of faith if you will, but I wanted a personal encounter with the Creator of the Universe.
And it happened. August 8, 2000, at Stoneleigh Bible Week, there was a call for people who wanted a fresh filling of the Holy Spirit to come forward for prayer. I went forward, not expecting much. I knew what I thought would happen, as I had seen it happen before - someone would stand by me and pray that I would have a fresh encounter with God, and after a few minutes I would thank him or her politely and leave, feeling unfulfilled. In the meantime, the more extrovert Christians in the groups would be moaning and falling over around me.
This time, a small, nerdy-looking man from some church in the south of England offered to pray with me. He started speaking out seemingly insipid requests, not inspiring me with much confidence for the "session". I tried to concentrate on ignoring him and focusing my attention on God.
After a couple of minutes, I felt what seemed to be my mind saying "I think I should kneel". So I went to kneel down, and whammo! Flat on my back with no memory of actually falling, with an intense feeling of peace and love surrounding me and a complete inability and unwillingness to get up, and a total lack of self-preservation instinct given the hundreds of people milling around my inert form. I felt, and I truly believe that I was in the presence of God at this point. After what I was told was about ten minutes, I felt the sensation ease up, and I was able to stand, although I couldn't stop shaking for most of the afternoon.
Possible explanations? As I've said, I am completely convinced it was God showing me His power and love, as I had asked Him to. Read Acts 2. You could try to claim it was some form of hypnosis, but I've seen it happen many times and am convinced it is not. I don't believe it has much to do with the person praying; nor was I expecting to fall. Fainting? Nope - lots of people were on the floor, and I have never fainted in my life.
So when I appear to write write-ups whose content appears to assume the existence of God as a given fact, the reason is that to me, it is a given fact. This isn't something I can prove to you, you'll have to find it for yourself. By all means add write-ups challenging this, but I know what I know. Given humanity's problem with absolute truth, I expect some backlash!