Select the most appropriate 'one word' for the expression given below.

One who is against religion.

Heretic

The most appropriate one-word substitute for the expression "one who is against religion" is heretic.

• Poacher means someone who illegally hunts or fishes on private property.
• Lunatic means someone who is mentally ill.
• Bandit means a member of a gang of robbers or outlaws.

Heretic means someone who holds religious beliefs that are considered to be unorthodox or unacceptable by the established church or religious authorities. It is the most appropriate word for the given expression because it specifically refers to someone who is against religion.