WordNet records no synonym and no antonym for confession. It is the only member of each of its synsets, 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.
an admission of misdeeds or faults
a written document acknowledging an offense and signed by the guilty party
a public declaration of your faith
the document that spells out the belief system of a given church (especially the Reformation churches of the 16th century)
Synonyms, antonyms and the broader and narrower relations for confession come from Princeton University’s WordNet. Full sources and licences.