fanatic has 4 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a person motivated by irrational enthusiasm (as for a cause)
“A fanatic is one who can't change his mind and won't change the subject”
marked by excessive enthusiasm for and intense devotion to a cause or idea
“rabid isolationist”
Synonyms, antonyms and the broader and narrower relations for fanatic come from Princeton University’s WordNet. Full sources and licences.