Atheist

Atheist comes from Greek, from a- meaning "not" or "without" and theos, meaning "god." In English is simply means "a person who believes that God does not exist."

https://freeglobaluniversity.blogspot.com/search/label/Atheism

Agnostic

 Agnostic comes from the Greek word meaning "unknown" or "unknowable" (a-, "not" or "without," and gnōstos, meaning "known"). It means "a person who does not have a definite belief about whether God exists or not" 

https://www.google.com/search?q=Agnostic