Atheist:  "I'll believe in God when you can prove that your god exists."

Answer:  If by "prove" you mean experiment, measure, and observe (via the scientific method) God's absolute truth, beauty, personality, omniscience, omnipotence, sovereignty, holiness, righteousness, justice, love, mercy, faithfulness, patience, and perfection (i.e. if you expect to observe and measure that which is unobservable and unmeasurable) then NO we cannot "prove" via natural means that who by definition is Supernatural.  However, we can demonstrate that belief in God; more specifically, the Christian God is not only logical and reasonable, but is the most plausible explanation for reality (philosophy, science, history, and archaeology) the way we know it.

Click here for 6 reasons why we believe in a personal God.