Got it! The Latin for unfaithful is 'infidelis'.(obvious really: think 'infidel' !) In late and ecclesiastical Latin 'infidelis' was used to describe anyone who did not have the Christian faith, a non-believer..
So we can use 'infidelibus' instead of 'qui non credentibus' in cancross' version and 'illibus qui non credunt' in QM's. Infidelis is an adjective, not a noun, but that doesn't matter because 'people' or 'those' would be understood.It would be taken to mean 'those people who are'without faith', 'non-believers'.You can't have a thing which is 'without faith' a 'non-believer', only a person, so the adjective could only be referring to people.