By your logic every human on Earth is agnostic because it's impossible for any experiment to prove or disprove whether God or any other religious or supernatural being(s) exist. People just have to decide what they want to believe, whether it's a specific religion, a lack thereof, or a "spiritual but not religious" kind of thing.
Personally, I just accept people's labels for themselves. If a person says he's Christian, atheist, agnostic, Jewish, whatever, I'm take him at his word and don't try to say "well, uh, actually I say you're not!!!1"
Hey, words mean something. Again, You just don’t understand Christianity. It is that the Supreme Being is accepted as an article of Faith whereas Atheism is having knowledge there is no Supreme Being (which he cannot possess). You seem to be deliberately misunderstanding.