One who does not believe in existence of god
A). Egoist
B). Atheist
C). Stoic
D). Naive
Atheist
One word substitute is Atheist.
Egoist: a self-centered or selfish person.
Atheist: a person who disbelieves or lacks belief in the existence of God or gods.
Stoic: a person who can endure pain or hardship without showing their feelings or complaining.
Naive: showing a lack of experience, wisdom, or judgement.