I think William could redefine the role?
Just musing here <ARRIVAL IN WANKERTON ALERT> but as pp have said elsewhere, Henry VIII was given the title 'Fidei Defensor' by the (then) Pope Leo when he (H8) was still a loyal Roman Catholic, and had written a treatise opposing the teachings of Martin Luther.
'FD' later stayed with the monarch and morphed into a linkage with the title of Supreme Governor of the Church of England. This was solidified through imagery on coinage - powerful stuff to spread through an Empire, later Commonwealth.
The literal translation of Fidei Defensor from the Latin, which has no definite article, could be 'Defender of Faith' which Charles prefers, aiming to protect all faiths in Britain & Realms rather than only the Church of England.
I think with regard to the translation, the Pope would hardly have rewarded Henry VIII for other than defending Roman Catholicism, so probably is should be translated as 'Defender of the Faith'; but Charles understandably is trying to make a mediaeval anachronistic title make sense.
IMO the 'Head of the Church' should defend all Christian denominations and the Abrahamic faiths, and support religion and belief as being protected thought and speech, including all forms of agnosticism and atheism (ie not believing in any particular religions, beliefs and ideologies), which is where our law seems to be.
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