I have a thought in my head right now - can anyone tell me what it is? I say this not to mock, but to raise the limitations of human knowledge and experience. You may make intelligent guesses at what I could be thinking, even possibly use some brain imagining techniques to help, but truth is that you can't know my thoughts, unless I choose to reveal them to you. 

Now if this is true at the human relational level, how much more so at a divine level? You see, the bible aside, those who say outright that God exists or doesn't exist are starting with the same assumption - that we can determine this on our own. It's presuming that we somehow have the capacity to answer a question like "does God exist?". But if I can't even tell your thoughts without you revealing them to me first, how can I presume that on my own, I can determine the very thoughts (or existence) of God?

What then, would prove the existence of God? Well, divine disclosure. Here's my main point:

The only way we can know if God exists is if he chose to reveal himself to us.

Here's the important news. Christians believe that God has revealed himself - through the bible, and through the person of Jesus. The bible was written by man, yes, but under the guidance and leading of God. Its validity and reliability has come under a lot of attack recently - but not meaning to simply brush those aside, why don't you give it a read for yourself? Test it. If it actually is the word of God, see if it speaks to you. Test the person of Jesus, and all the things he did. His miracles, his words, his teachings, his death and his resurrection, and the way it changed so many peoples' lives.

In summary, don't shut the door to God without even having listened to him. Read the bible for yourself.