The atheist says that he does not believe that God exists.
God says (in His Word) that atheists do not exist.
Who do you think is right?
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness;
Because that which may be known of God is manifest in them; for God hath shewed it unto them.
For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.