He has also, like all previous British Monarchs, taken as one of his royal titles "Defender of the Faith". Defense against what or whom, one might pause to consider?
Well, the history of the title might be illuminating: It was awarded to Henry VIII by the pope (Leo X), in 1521, in recognition of Henry's defending the Catholic Church against Martin Luther. Henry wrote a book
Assertio Septem Sacramentorum, which defended, amongst other things, the sacrament of marriage and the supremacy of the pope as head of the Christian faith.
So to answer your question,
Protestantism.
When Henry bailed on the pope, in a fit of pique about being refused a divorce, one of his ways of sticking a finger up at the Vatican was his refusal to give up the title 'Fidei Defensor'.
The whole thing is massively ironic. English monarchs have used 'FD' as a title ever since, despite being the heads of a major Protestant church. But THE faith (singular) to which the title refers is Roman Catholicism, a faith that the English Monarch is legally prohibited from holding.