Do I know god? I would say yes, I do. I know what the notion of god is. It is an entity the believers claim to have created everything. God has lots of love, but like blood and burn offerings. Does that make god real?
Am I sure god does not exist? No I don't need to. I am not sure whether pink unicorn exists or not. I am not sure whether fire blowing dragon exists or not. Do I have to explicitly seek knowledge to verify an unverifiable claim - by its definition, non-existence entity cannot be verified. So before anyone have valid proof of the existence of god, god does not exist.
Agnostic is an unnecessary position in light of the lack of proof for those who claim god exists. I find taking such a position is giving the religious too much benefit of doubt. Like anything, why god needs to be an exception when there is a demand of proof? No proof, no god.