Step 1: Understand the meaning of the given word.
An atheist is a person who does not believe in the existence of God.
Step 2: Identify the opposite meaning.
The opposite of an atheist would be a person who believes in God or follows a religion.
Step 3: Examine the options.
Skeptic: one who doubts — not necessarily religious
Pagan: follower of a non-mainstream religion — not opposite
Disciple: a follower or believer, especially of a religious teacher — opposite ✔
Agnostic: one who is unsure about the existence of God — not opposite
Step 4: Final conclusion.
The word opposite in meaning to atheist is disciple.