atheist:
(plural: ‘atheists‘; the word opposite in meaning: ‘theist’)

R.I.P. Sigmund Freud, psychologist & atheist
a person who does not believe or does not accept that there is God;
a disbeliever; a nonbeliever,

an atheist advertising himself
e.g.
“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.)
For more on atheism, please click here.
Clarification: An ‘atheist’ is a person who does not believe in God, a ‘theist’ is a person who believes in God and an ‘agnostic‘ is a person who thinks that it is not possible to say that there is a god or there is no god. A ‘nihilist‘, on the other hand, does not believe in the truth of anything!
