(plural: ‘atheists‘; the word opposite in meaning: ‘theist’)
a person who does not believe or does not accept that there is God;
a disbeliever; a nonbeliever,
“An atheist is a man who has no invisible means of support.” (John Buchan ‘On Being a Real Person, from thefreedictionary.com)
There is always a conflict between an atheist and a theist or believer, and either one comes up with some logic and evidence.
Just saying “Oh, God!” does not make an atheist a theist or believer. (It’s just an expression of surprise.)