Step 1: Read the description carefully.
The clue is: a person who opposes established beliefs and traditions. The key idea is going against accepted doctrines.
Step 2: Recall the matching word.
A person who rejects or challenges widely accepted beliefs, especially in religion, is called a heretic.
Step 3: Check option Fanatic.
A fanatic has extreme enthusiasm for a belief and usually supports it strongly, rather than opposing it. So this is wrong.
Step 4: Check option Disciple.
A disciple is a loyal follower of a teacher or doctrine. This is the opposite of opposing beliefs, so it is wrong.
Step 5: Check option Martyr.
A martyr suffers or dies for a cause or belief. This is about sacrifice, not about opposing traditions, so it is wrong.
Step 6: Pick the matching word.
Heretic is the word for a person who opposes established beliefs and traditions, which matches the clue exactly. \[ \boxed{\text{Heretic}} \]