don't know if there is a god" is not answering the question "do you believe in god?". it's simply not answering it, because again, you don't need to know something is true in order to believe in it. that's not what was asked. the question was not "do you know for a fact there is a god?"
same thing applies in any other department with the word "belief". i may not "know" whether you can actually win the football game, but i "believe you can win". i may not know i'm in fact going to win the lottery, but "i believe i'm not gonna win". sometimes, my belief may be backed 