sincerity has 4 distinct synonyms spread across 3 of its 4 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. Its one recorded opposite is insincerity.
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 quality of being open and truthful; not deceitful or hypocritical
“his sincerity inspired belief”
Synonyms, antonyms and the broader and narrower relations for sincerity come from Princeton University’s WordNet. Full sources and licences.
a quality of naturalness and simplicity
“the simple sincerity of folk songs”
the trait of being serious
“a lack of solemnity is not necessarily a lack of seriousness”