The logical conclusion that something outside the natural world (God) must exist in order for the universe to exist is not a satisfactory one for skeptics. Their likely response to this line of reasoning will be, “Then what created God?” This might appear to put the believer in a corner, but let’s look at two reasons why it does not:
#1) The skeptic is fixing the terms of the argument. He assumes his worldview, naturalism, is already the correct one and that we must explain God in naturalistic terms. It’s like saying, “I only believe in squares, but I’ll believe in circles if you draw me a square one.” You will not be able to explain God in the naturalistic terms he demands so your answer will be dismissed.
#2) The question itself is invalid b/c it confuses two categories of things, those that are contingent (need a cause) and those that exist independently. Causes do not apply to a self-existent being, yet this being must exist independently in order for any natural thing to exist.
Point out that he’s asking you for the impossible. Then ask some questions of your own such as, “If God doesn’t exist, what law of physics allows the universe to spring from nothing?;” “What biological laws allow life to spring from non-life?;” or, “How much faith is required to believe that?”