" The Oxford, Chambers, Websters and the online 'Free Dictionary'"
Oxford dictionary:
http://oxforddictionaries.com/definition/english/atheist?q=atheist
"a person who disbelieves or lacks belief in the existence of God or gods"
This definition would include a newborn baby - because it includes both those who simply lack a belief and those who choose disbelief.
Merriam-Webster:
http://www.merriam-webster.com/dictionary/atheist
"Definition of atheist
: one who believes that there is no deity"
This one would not.
Cambridge Dictionaries online:
http://dictionary.cambridge.org/dictionary/british/atheist?q=atheist
"someone who believes that God or gods do not exist"
This one would not.
dictionary.com:
http://dictionary.reference.com/browse/atheist?s=t
"a·the·ist [ey-thee-ist]
noun
a person who denies or disbelieves the existence of a supreme being or beings."
This one would not.
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Personally, I'm a little uncomfortable with any labels being bestowed on newborns - it's a bit like referring to a believing or nonbelieving housecat. Neither side can really 'claim' it.
And besides, even if a newborn believes something - that does not mean it is true.