Step 1: Understanding the definition.
A person who does not believe in the existence of God is called an atheist.
Step 2: Analysis of options.
- Egotist: A person who talks too much about themselves.
- Altruist: A person who works for the welfare of others.
- Atheist: A person who denies the existence of God.
- (Emigrant: A person who leaves their own country to live in another.
Step 3: Conclusion.
Hence, the correct answer is atheist.