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."
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company
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atheist \A"the*ist\,
n.
1. One who disbelieves or denies the existence of a God, or supreme intelligent Being.
: Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.