WordNet records no synonym and no antonym for ecumenical council. 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.
(early Christian church) one of seven gatherings of bishops from around the known world under the presidency of the Pope to regulate matters of faith and morals and discipline
“the first seven councils through 787 are considered to be ecumenical councils by both the Roman Catholic church and the Eastern Orthodox church but the next fourteen councils are considered ecumenical only by the Roman Catholic church”
Synonyms, antonyms and the broader and narrower relations for ecumenical council come from Princeton University’s WordNet. Full sources and licences.