This news item reminded me of a bee in my bonnet:
A major new Evangelical Alliance report on the relationship between church and society has urged Prince Charles to abandon ideas of re-titling himself ‘Defender of Faith’. Instead, Faith and Nation encourages him to adopt the exclusively Christian title of ‘Defender of the Faith’.
'Fidei Defensor' is the title awarded by the Pope of the time to King Henry VIII. The Pope clearly intended Henry to be commended for keeping the unreformed faith, after he had published a book defending, if I remember, the medieval doctrine of the Mass.
When Henry threw in his lot with the reformers, he kept the title, which he and others ever since have re-interpreted as meaning Defender of the reformed Faith.
But the title is more versatile yet. It is only custom that has led everyone to translate it as 'Defender of THE Faith.' Latin, having no definite article, leaves the matter open.
Let Prince Charles take the Coronation Oath in Latin. The new King will have made no change in the traditional wording. Then both he and the Evangelical Alliance can be happy.