I'm going to be a contrarian (and pedant) and disagree with the general consensus that rocks and babies are atheists. They are non-theists, certainly, but also non-atheists. The term atheist was coined before the word 'theist'. We often assume it comes from 'a-theist', or 'not a theist'. However it was originally constructed as 'athe-ist'. 'Without God' plus the suffix 'ist', which indicates a person concerned with or interested in whatever precedes it.
An atheist is a person capable of considering the proposition that deities exist, who does not accept that proposition.
An atheist is a person capable of considering the proposition that deities exist, who does not accept that proposition.
I'm not anti-Christian. I'm anti-stupid.