pledge has 7 distinct synonyms spread across 5 of its 9 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.
give as a guarantee
“I pledge my honor”
bind or secure by a pledge
“I was pledged to silence”
a deposit of personal property as security for a debt
“his saxophone was in pledge”
someone accepted for membership but not yet fully admitted to the group
a binding commitment to do or give or refrain from something
“an assurance of help when needed”
Synonyms, antonyms and the broader and narrower relations for pledge come from Princeton University’s WordNet. Full sources and licences.