Exactly--defender of the faith? Does he know which faith he's charged with defending?
The Monarch is Defender of the Faith, that is, Defender of protestant anglicanism. That excludes catholics, jews, muslims, hindus etc.
Charles has already said that he intends to be 'Defender of the Faiths' to reflect the wider range of religious beliefs in the UK and to encompass the catholic, the jew and the muslim amongst others.