Read the book "The Case for Christ" by Lee Strobel. And it's faith. When you sit down on a chair, you have to have faith that it won't collapse under you. We have a faith in God. I'm not really sure how to explain this, but the book above is excellent:)
You don't know. That's where the word "faith" comes in. Believing in something you cannot see. You can't see the air, but it is there. Each of us has a choice. You can choose to believe or not; I choose to believe.