What does Prince William actually believe, asks CATHERINE PEPINSTER? The Christian faith underpins his father, King Charles, and holds our monarchy together…

With Ash Wednesday tomorrow marking the start of Lent, the King's thoughts will no doubt turn to an important moment in a Monarch's year and an occasion he will not want to miss.

An ancient Christian ceremony that combines leadership and service. During next month's Maundy Thursday service, the king will wash the feet of his subjects and hand out specially minted White Coins.

It has deep roots.

Dating back to the Middle Ages, the White Service commemorates the Last Supper, when Christ washed the feet of his disciples, but also remembers the promise to serve made in the king's coronation vows.

It is no coincidence that Handel's Zadok the Priest – heard at every coronation since that of George II – is sung every year.

But with King Charles undergoing treatment for cancer, there's a good chance he won't be there this year (the location has yet to be announced) leaving that duty to – most likely – his heir, Prince William.

This task would certainly remind the prince of his fate – and how closely that future is tied to the Christian faith.

It would also raise a number of questions that linger a little more persistently every year: What exactly does Prince William believe?

And how committed he is to the Church of England he will one day lead.

There can be no doubt about his father, King Charles, just as there was no doubt about his grandmother, the late Queen Elizabeth.

Charles' considerable faith is very real and has sustained him over the years.

Amid the considerable coverage given over the years to the king's interest in other religions, especially Islam and Judaism, his own strong Christian beliefs have sometimes been overlooked.

This is a mistake. Charles prays often. He uses openly reverent language, for example referring to “Our Lord Jesus Christ” rather than just “Jesus.”

He had his own chapel built in the grounds of his home, Highgrove House in Gloucestershire.

Some of the greatest influences on him are devout believers, including the former Bishop of London, Lord Chartres, the Bishop of Southwark, the late Mervyn Stockwood, the Chaplain of Jungian and Cambridge, the Reverend Harry Williams and the poet Kathleen Raine.

Despite some early questions about what the king would call himself at his coronation, he chose to be Defender of the Faith.

When he first addressed the nation after his accession to the throne, he used these words: “I solemnly pledge myself for the remaining time that God grants me.”

Eight months later, he was greeted at his coronation by a choir boy – the youngest person there – who stepped forward and said, “Your Majesty, as children of the kingdom of God, we welcome you in the name of the King of Kings.

And Charles replied, “In his name and after his example I come not to be served, but to serve.”

The comfort of faith was something his mother, Elizabeth II, also knew.

She was an avid reader of the Bible and knew the Anglican Book of Common Prayer intimately, and her most treasured possessions included a special prayer book prepared to help her prepare for her 1953 coronation by the then Archbishop of Canterbury , Geoffrey Fisher.

As she was dying, a preacher read the Bible to her.

While Elizabeth II had a simple Anglican faith, supported by a lifetime of Sunday church attendance, her son's interest in religion is more like that of his father, Prince Philip – full of curiosity and a sense of adventurous exploration.

His great friend, Lord Chartres, once told me that the king is 'captivated by religion'.

That fascination does not seem to be shared with the Prince of Wales.

There is little evidence of his own religious beliefs, although he was baptized as an infant and later confirmed by Lord Chartres.

By the time Charles was in his forties – the age William is now – he regularly gave long speeches on religion and ethics.

Making speeches is not William's strong suit, that's true. He prefers practical action.

Where there is evidence of his interest in faith, it is in his strong ties with religious organizations that work at the grassroots and help the homeless. He is passionate about such things.

However, his commitment to charity has not been enough to prevent speculation that William would cut ties with the Church of England when his time comes. That he would not be Supreme Governor.

According to biographer Robert Hardman's new book, Charles III – New King, New Court, William is not a regular churchgoer and “instinctively feels uncomfortable in a faith environment.”

Not that the prince could unilaterally change the relationship, because the British monarchy and the established church, the Church of England, are so linked that he could not become king without being 'in communion' with the Church of England, according to the Deed of Settlement, 1701.

Constitutional experts Robert Hazell and Bob Morris recently wrote that:

'While correct in principle, removing the restriction on the monarch's freedom of belief would in practice raise questions about the newly changed constitutional status of the Church of England, together with the role of parliament and the monarchy in towards religion in general'.

Today and for the foreseeable future, the king's siblings and children will be more concerned about his health than the constitutional niceties of church attendance.

But whoever washes the feet of the 75 chosen citizens on March 28 – 75 in honor of Charles' age – will be unavoidable: the Christian faith, which was at the heart of the king's life and sought recovery, is at the heart of the life of the king. Monarchy itself.

At some point, answers will be needed.

• Catherine Pepinster is the author of Defenders of the Faith – the British Monarchy, Religion and the Coronation