The title of "Defender of the Faith" was first bestowed on Henry VIII by the pope, because of his defense of the Catholic view of the Eucharist against Martin Luther. Henry VIII later had Sir Thomas More and Bishop John Fisher executed because of their loyalty to the Catholic Church. It seems ironic that the non-Catholic monarchs of England have continued to use a title which came from the pope.
Since Latin doesn't have either a definite or an indefinite article, Defensor fidei could mean "defender of a faith"--so whatever faith the heir apparent has, he can be the defender of that.
Excuse me, but I myself am quite thankful for one particular dalliance between William the Conqueror's father and mother.