WordNet records no synonym and no antonym for antipope. It is the only member of its synset, 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.
someone who is elected pope in opposition to another person who is held to be canonically elected
“the antipopes resided in Avignon during the Great Schism”
Synonyms, antonyms and the broader and narrower relations for antipope come from Princeton University’s WordNet. Full sources and licences.