Atheism is the lack of belief in god(s), period. Most atheists do not fear "that there may be no God." They don't consider it a reasonable possibility. While some atheists do, there is no philosophical or intellectual imperative to deny the existence of "God." Lack of belief is sufficient.
'Deity' is basically a fancy word for God or other divine being. Atheist means without theism or religion. So an atheist is somebody who does not believe in god. They do not believe in any deities at all. The reason an atheist does not consider the existence of gods a possibility is that there is no evidence for any. Atheists, like all people everywhere should, take that last sentence and analyse it and realise that therefore a god cannot be considered to be real. Anything can only be considered to be real if there is evidence. Short answer: There are no atheist deities by definition.
"John is an atheist, which means he does not believe in the existence of any gods or deities."
David is an atheist because he knows that the existence of god cannot be proven.
Atheist
Atheist would be capitalized only if it were part of somebody's name, for example if somebody referred to himself as Atheist Bob. However, in the sentence "I saw that atheist Bob at the bank today," atheist is not capitalized because it's not part of Bob's name.
My friend is an atheist and does not believe in the existence of any gods.
No he wasn't. He did push for sociology to be considered a science, however that does not mean a person is atheist.
Is considered an atheist by the mainstream.
No definitely not. He is famous for attempting to prove Gods existence through his ontological arguments.
Yes; he describes himself as completely a-religious.
Atheist - If he is sure there there is no specific god because of the impossibility and/or improbability of his existence. Agnostic - Not sure whether there is a god or not. Usually does not care. An atheist is the opposite of a theist. The theist believes there is evidence for the existence of their favorite god. The atheist does not. It would be illogical for the atheist to "deny" the existence of something for which there is no evidence. Atheism is merely the lack of a belief. Knowledge is absolute. Since nobody can know with absolute certainty no gods exist, everyone is agnostic. So the so-called "agnostic" has the same lack of knowledge as the atheist or theist has. It's just commonly accepted that an agnostic does not commit either way.
You may be thinking of Thomas Paine; he was widely considered an atheist, although he was probably a Deist.