Unbeliever (noun)

A person who does not believe in a particular religion, doctrine, or system of beliefs; a skeptic.

Origin:

From the word believe meaning "to accept as true" and the prefix un- meaning "not".

Examples:

1. He was an unbeliever, who did not believe in God.
2. She was an unbeliever, who did not believe in the afterlife.
3. Many people consider themselves unbelievers in regards to astrology.
4. The society was not tolerant of unbelievers.
5. The debate between the believer and the unbeliever was heated.