Uhhh ... the faith (Christianity,) not the faithful.
<< He suggested the title held by all British monarchs since Henry VIII implied they would protect only Christians. >>
The bastard's mad.
Fortunately so are all of his sometime to be subjects, so none will notice.
Meanwhile "The Faith" that the British monarch defends is specifically the Holy, Catholic and Apostolic [Anglican] Church of England of which the monarch is the titular head.
Christianity has done pretty darned well these 2000 odd years and will continue to despite that dopes like Charly Battenberg Windsor get to stuff around with bits of it now and again.
Meanwhile, before Batty Charly gets his paws on it, in any given week/month/year approximately ten times more people regularly attend mosque in London than regularly attend Church of England services.