Proof of a negative statement is difficult to pull off. For example, how would you prove the negative assertion “There is no gold in Alaska”? You would have to determine the limits of Alaska, its borders and depth and height, then dig up every cubic inch of Alaska. If there was one cubic inch you did not dig, there still might be gold there. On the other hand, how would you prove the positive assertion, “There is gold in Alaska”? Easy — you need find only one piece.
Similarly, what would you have to know in order to know for sure that there is no God? You would have to know everything. If there was one thing you did not know, that one thing might be God. We are so far from knowing everything that there is to be known, that the dogmatic assertion “There is no God” is not only not provable, it is also arrogant.
from C. S. Lewis’s Case for Christ by Art Lindsley