Concept: This question asks for a specific term that describes a person based on their belief (or lack thereof) in God or gods.
Step 1: Defining the terms in the options
(1) Theist: A person who believes in the existence of a god or gods, especially a creator who intervenes in the universe. (From Greek "theos" meaning god).
(2) Atheist: A person who disbelieves or lacks belief in the existence of God or gods. The prefix "a-" often means "not" or "without". (a-theist: without god).
(3) Religious: Relating to or believing in a religion. While many religions involve belief in God or gods, being religious is a broader term that encompasses practices, rituals, and a system of faith. One could be religious without a traditional theistic belief (e.g., some forms of Buddhism).
(4) Irreligious: Indifferent or hostile to religion, or having no religious beliefs. This is related to religion as a system, not specifically to the belief in God, although there's often overlap with atheism.
Step 2: Matching the definition
The definition given is "One who does not believe in God".
This directly matches the definition of an Atheist.