A person not sure of the existence of god

One word substitute is Agnostic.

Theist: a person who believes in the existence of a god or gods, specifically of a creator who intervenes in the universe.
Atheist: a person who disbelieves or lacks belief in the existence of God or gods.
Agnostic: a person who believes that nothing is known or can be known of the existence or nature of God.
Cynic: a person who believes that people are motivated purely by self-interest rather than acting for honourable or unselfish reasons.