Wrong. Merriam-Webster Online fails to resolve the disagreement.
M-WO defines atheist as "one who denies the existence of God" (DrC's exact words which you dispute as being an authorititive definition) and atheism as a) a disbelief in the existence of deity or b) the doctrine that there is no deity. In OWK's and Dimensio's favor, the a) definition takes precedence when the word is "atheism."