WordNet records no synonym and no antonym for syncretism. 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.
the union (or attempted fusion) of different systems of thought or belief (especially in religion or philosophy)
“a syncretism of material and immaterial theories”
the fusion of originally different inflected forms (resulting in a reduction in the use of inflections)
Synonyms, antonyms and the broader and narrower relations for syncretism come from Princeton University’s WordNet. Full sources and licences.