i think a couple of things need clearing up.
Atheism - you do not believe in any God or gods.
Agnosticism - you are not sure whether there is or isn't a god, whether that is because you haven't thought about it, don't much care or cannot make a judgement due to limited evidence doesn't matter.
Theist - you believe in a God or gods
Religious person - someone who follows a religion and it's teachings
When you look at it, you can see that it is possible to be a theist without being religious, it is perfectly fine to say that you are not religious but you believe in a god.
Another thing is that it is virtually impossible to prove that something doesn't exist, saying "prove that there is no god" is like saying "prove that there are no unicorns", i can never disprove the existance of unicorns - i assume that they do not exist because i have never seen one or seen any compelling evidence to suggest that they do exist, but it is impossible to be sure that on another planet in another galaxy, there don't exist four legged animals with horns. the same can be said for God, we can't disprove his existence, only look at the evidence and make a personal judgement.