Bona fides looks like a plural word in English, but in Latin it is a singular noun that literally means "good faith." When bona fides entered English, it at first stayed very close to its Latin use, and it also kept its singular form—for example, "a claimant whose bona fides is unquestionable."

(italics) Latin. (used with a singular verb) good faith; absence of fraud or deceit; the state of being exactly as claims or appearances indicate: Compare mala fides. The bona fides of this contract is open to question. All our bona fides are on file with the SEC.