faithful has 3 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. Its one recorded opposite is unfaithful.
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.
steadfast in affection or allegiance
“years of faithful service”
not having sexual relations with anyone except your husband or wife, or your boyfriend or girlfriend
“he remained faithful to his wife”
Synonyms, antonyms and the broader and narrower relations for faithful come from Princeton University’s WordNet. Full sources and licences.
a group of people who adhere to a common faith and habitually attend a given church