confirm has 6 distinct synonyms spread across 2 of its 5 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.
make more firm
“Confirm thy soul in self-control!”
support a person for a position
“The Senate confirmed the President's candidate for Secretary of Defense”
administer the rite of confirmation to
“the children were confirmed in their mother's faith”
Synonyms, antonyms and the broader and narrower relations for confirm come from Princeton University’s WordNet. Full sources and licences.