WordNet records no synonym and no antonym for fundamentalism. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 form of Protestantism that takes the interpretation of every word in the sacred texts as literal truth
the strict adherence to the basic principles or doctrines of a religion, ideology, etc.
Synonyms, antonyms and the broader and narrower relations for fundamentalism come from Princeton University’s WordNet. Full sources and licences.