Heidi, Heidi, Heidi,
You will find few, if any, that 'claim' to be Christians that will answer this with anything other than that they 'know' Him. And anyone that questions His existence is CERTAINLY NOT a Christian. Impossible to question His existence and at the same time, allow Him into your heart.
Now, the REAL question, how many out there claim to 'know' Christ, but have NEVER even met Him? Answer that one and then we will have a basis to seperate Christians from those that 'just talk about it'.