One who believes that everything is pre-destined

One word substitute is Fatalist.

Pessimist: a person who tends to see the worst aspect of things or believe that the worst will happen.
Fatalist: someone who feels that no matter what he or she does, the outcome will be the same because it's predetermined.
Palmist: a person who practises palmistry; a palm reader.
Astrologer: a person who uses astrology to tell others about their character or to predict their future.