Select the option that can be used as a one-word substitute for the given group of words.
One who believes in the existence of God

The correct option is Theist.

Let's understand the meaning of the given words to see why "Theist" is the correct one-word substitute:

1. Theist: A person who believes in the existence of a god or gods. This directly matches the given group of words "One who believes in the existence of God".

2. Atheist: A person who disbelieves or lacks belief in the existence of God or gods (the opposite of a theist).

3. Fatalist: A person who believes that all events are predetermined by fate and therefore inevitable.

4. Protagonist: The main character or a leading figure in a story, play, or novel.

Therefore, the word that best substitutes "One who believes in the existence of God" is Theist.