It is impossible to prove a negative.
Ridiculous. I can prove you aren't dead. I can prove I'm not dead.
I can prove I don't have four kids. I can prove I don't have a beak. I can prove Katie Couric is not a Republican.
Proving negatives even has a word associated with it: Disprove.
Atheists are sure God doesn't exist. There isn't room for doubt, else they'd be fine with the appellation agnostic. They aren't fine with that.
OK, that means atheists are sure beyond doubt. As a Christian, I freely admit the role of faith in my positions on Christ. Atheists don't admit the same. They are sure. Good, let's see the evidence and then perhaps I can have my Sunday mornings back. I don't care if they find Christ, Apollo, or Shiva, but they can't tell me they are sure of ANYTHING's non-existance unless they supply proof. This includes the Easter Bunny, or anything else.
In the absence of evidence, only faith exists among adherents to anything. As such, atheism is more a belief system than Christianity is, when you consider that they admit no doubt as to the non-existance of God. There is no spectrum of intensity to their position - just one setting - God doesn't exist.
Everyone else adheres to some sort of spectrum of belief, from "There must be intelligent design to things" to "God made the creation in seven literal 24 hour days" for example.