At the moment of his official accession, King Charles III stated his constitutional responsibility as 'Defender of the Faith'.
Two decades ago, there had been some speculation that on his accession to the throne Charles would use the title 'Defender of Faith' after he described himself as 'faith's defender'.
However on Saturday, making an oath relating to the Church of Scotland, he said:
"I, Charles III, by the grace of God of the United Kingdom of Great Britain and Northern Ireland and of my other realms and territories, King, Defender of the Faith, do faithfully promise and swear that I should inviolably maintain and preserve the settlement of the true Protestant religion as established by the laws made in Scotland in prosecution of the Claim of Right and particularly by an act intituled an act for securing the Protestant religion and Presbyterian church government and by the acts passed in the Parliament of both kingdoms for union of the two kingdoms, together with the government, worship, discipline, rights and privileges, of the Church of Scotland."
The King is also now Supreme Governor of the Church of England.
Despite some years ago describing himself as 'faith's defender', in 2015 the then Prince of Wales clarified his earlier remarks in a BBC radio interview:
"No, I didn't describe myself as a defender: I said I would rather be seen as 'Defender of Faith', all those years ago, because, as I tried to describe, I mind about the inclusion of other people's faiths and their freedom to worship in this country. And it's always seemed to me that, while at the same time being Defender of The Faith, you can also be protector of faiths. It was very interesting that 20 years or more after I mentioned this - which has been frequently misinterpreted - the Queen, in her Jubilee address to the faith leaders, said that as far as the role of the Church of England is concerned, it is not to defend Anglicanism to the exclusion of other religions. Instead, the Church has a duty to protect the free practice of all faiths in this country. I think in that sense she was confirming what I was really trying to say - perhaps not very well - all those years ago. And so I think you have to see it as both. You have to come from your own Christian standpoint - in the case I have as Defender of the Faith - and ensuring that other people's faiths can also be practised."
The title of Defender of the Faith was bestowed upon King Henry VIII in 1521 by Pope Leo X. After breaking away from the Catholic Church in order to divorce his first wife Catherine of Aragon, he declared himself Supreme Governor of the Church of England.
In his televised address to the nation following his mother's death on Thursday, King Charles also spoke of his own faith:
"The role and the duties of monarchy also remain, as does the sovereign's particular relationship and responsibility towards the Church of England - the church in which my own faith is so deeply rooted.
"In that faith, and the values it inspires, I have been brought up to cherish a sense of duty to others, and to hold in the greatest respect the precious traditions, freedoms and responsibilities of our unique history and our system of parliamentary government.
"As the Queen herself did with such unswerving devotion, I too now solemnly pledge myself, throughout the remaining time God grants me, to uphold the constitutional principles at the heart of our nation."
The role of Supreme Governor of the Church of England enables the monarch to formally appoint high-ranking members of the church on the advice of the prime minister of the United Kingdom, who is in turn advised by church leaders, such as the Lords Spiritual.
During her 70 year reign, the late Queen Elizabeth II appointed seven Archbishops of Canterbury and numerous bishops and deans.