Select the option that can be used as a one-word substitute for the given group of words.
One who does not believe in God

The correct answer is Atheist.

Let's break down the meaning of the given phrase and analyze the options to find the correct one-word substitute:
The given group of words is: "One who does not believe in God".

Let's examine each option:

• Atheist: This term comes from the Greek prefix 'a-' (meaning 'without') and 'theos' (meaning 'god'). Therefore, an atheist is a person who does not believe in the existence of a god or gods. This perfectly matches the definition provided in the question.
• Catholic: This refers to a member of the Roman Catholic Church, which is a Christian denomination. A Catholic definitely believes in God.
• Priest: This is an ordained minister of an Christian, Buddhist, Hindu, or other religion, authorized to perform certain rites and administer certain sacraments. A priest believes in and serves God.
• Reverent: This is an adjective that describes feeling or showing deep and solemn respect. It does not refer to a person's belief system regarding God directly, though it is often used in religious contexts.

Therefore, the appropriate one-word substitute for "One who does not believe in God" is Atheist.