Let X be your return if you don't believe in God. It has the following distribution:
X = whatever, with probability 1-p
X = going straight to hell (negative infinity), with probability p
where p=probability that God exists
Now calculate the expectation. Notice that, if God exists, no matter how small p is, your expectation will be negative infinity. (We are sure this "whatever" is not positive infinity.)
So, you don't need to believe in God only when you 100% sure that God does not exist. But are you that sure?