Person who does not believe in the existence of God:

A Theist
B Heretic
C Atheist
D Fanatic

Correct Answer: Atheist

The one word substitution is Atheist.

Theist: Belief in the existence of a god or gods, specifically of a creator who intervenes in the universe.
Heretic: A person believing in or practicing religious heresy.
Fanatic: A person filled with excessive and single-minded zeal.
Atheist: A person who disbelieves or lacks belief in the existence of God or gods.

Related Questions

One who does not believe in existence of god
A person not sure of the existence of god =
Person who believes that God is every thing and everything is god
My God! I am undone! 'My god' is-

Next steps