Close synonyms include adhere, bond, bind, stick, stick to. Each sense on this page lists its own synonyms, because a word with several meanings rarely has one substitute that works for all of them.
Is hold fast a common word?
hold fast is banded "Uncommon" — turns up in careful writing rather than conversation. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.
Definitions, synonyms and semantic relations for hold fast come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.