She was true to that vow. Her devotion to “Jesus Christ, Prince of Peace” was a fundamental and defining, though sometimes overlooked, pillar of her life.

Now, as her son Charles III takes over, he has by all accounts accepted the responsibilities of his religious titles without reservation. But he will bring a markedly different personal vision of religion and spirituality to the role.

What kind of monarch will King Charles III be? Different from his mum.

“The queen was very explicit about her Christian faith, but Charles’s is of a different nature,” said Ian Bradley, professor emeritus of cultural and spiritual history at the University of St. Andrews, who has written extensively about faith and the monarchy. “His is more spiritual and intellectual. Charles is more of a ‘spiritual seeker.’ ”

While the monarch’s authority within the church is largely ceremonial, it still matters. The king will formally approve all new bishops, for example. And pronouncements of the crown, especially on something as personal as faith in God, carry a special weight.

Particularly in her later years, Queen Elizabeth II was clear about expressing her beliefs, often citing the “guiding light” of Jesus, especially in her annual televised Christmas message watched by millions of people.

Many trace her shift in tone to her Christmas address of 2000, when she said, “For me, the teachings of Christ and my own personal accountability before God provide a framework in which I try to lead my life.”

The queen was sometimes referred to as the “last true believer,” said Stephen Bates, the Guardian newspaper’s longtime, now retired, religious affairs and royal correspondent. “She is the most religious sovereign since the [Protestant] Reformation” of the 16th century, he said.

While public assertions of faith are second nature — if not required — for U.S. leaders, they are unusual in Britain, a highly secular nation, where an aide to former prime minister Tony Blair once quipped, “We don’t do God.”

“We have a kind of unease about our politicians and our leaders expressing their faith, and to some extent this extends to the monarchy,” Bradley said. “It’s seen as un-British.”

Despite declining church membership and influence in daily British life, the monarch remains a powerful church symbol; British coins feature the queen’s likeness and letters in Latin that stand for, “By the Grace of God, Queen and Defender of the Faith.”

As his mother was, Charles is a regular churchgoer and clear that his faith is Christian. In his first address to the nation, on the day after the queen died, Charles cited his “responsibility” to the Church of England, “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,” he said. It was notable how quickly he placed faith into the context of the more secular “values” and “duty.”

In a 73-year lifetime of being a king-in-waiting, when he was able to speak more freely than he now can as monarch, Charles appeared to stake out a less doctrinaire religious and spiritual stance — even giving it its own title.

Charles said in a 1994 documentary that he was more a “defender of faith” than “the faith.” He questioned the impulse to prioritize one particular interpretation. “People have fought to the death over these things,” he said, “which seems to me a peculiar waste of people’s energy, when we’re all actually aiming for the same ultimate goal.” Instead, he said, he preferred to embrace all religious traditions and “the pattern of the divine, which I think is in all of us.”

When presented with the question again more than two decades later, he clarified his remarks, saying: “It’s always seemed to me that, while at the same time being Defender of the Faith, you can also be protector of faiths.”

The “Defender of the Faith” title dates to the 16th century, when it was granted by Pope Leo X to King Henry VIII for his defense of Catholicism. When Henry broke with the Catholic Church, he held on to the title, but now he was defending the Anglicanism of the Church of England.

Charles has long been an advocate for environmental causes, with a passion that Bradley described as “eco-spiritual.” In his 2010 book, “Harmony,” Charles issued a call for a “sustainability revolution” to reverse environmental threats to the planet, which he blamed in part on “the spiritual dimension to our existence” being “dangerously neglected during the modern era.”

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In the book, Charles took issue with “empiricism,” the view that since science cannot prove the existence of God, God must not exist. That kind of thinking, he wrote, “elbows the soul out of the picture.”

In an increasingly multicultural nation with a full rainbow of faiths, Charles has long expressed interest in and support for all forms of belief, particularly Islam and Judaism.

His mother also crossed new boundaries in that regard. She was the first British monarch to enter a mosque. Unlike predecessors, she met a succession of popes. On her 60th year on the throne, in 2012, she said the church “has a duty to protect the free practice of all faiths in this country.”

Pope Francis, as well as British Muslim, Jewish, Hindu and Sikh leaders, have all praised Elizabeth effusively since her death.

As the queen was sharing more about her faith, British society was becoming more secular.

According to the National Center for Social Research, church membership has dropped sharply over time, with only 12.5 percent of Britons in 2020 considering themselves members of the Church of England, down from nearly 36 percent in 1985. Of those who considered themselves Anglican in 2020, more than 40 percent said they “never” attend services.

Similar to the United States, British society has in recent years become less reliant on and structured around institutions that were once bedrocks of daily life. The center’s research showed that people who claimed “no religion” rose from 34.3 percent in 1985 to almost 49 percent in 2020.

As the number of worshipers drops, hundreds of historic churches have been taken out of service and turned into apartments, offices, pubs, spas, shops and even sporting centers with rock-climbing walls.

The church has changed in important ways, including a decision in 2002 to allow divorced people to remarry in the church. Three years later, Prince Charles and his longtime partner, Camilla Parker Bowles — both divorced — were married in a civil ceremony that was blessed immediately afterward in a chapel at Windsor Castle by the archbishop of Canterbury.

Now king, Charles is the first divorced monarch since Henry VIII — although two of Henry’s prolific string of marriages technically ended in annulment, not divorce.

It was not until 2018, when Charles’s son Prince Harry married American actress Meghan Markle in the same chapel where his father’s marriage had been blessed, that a royal wedding of a divorced partner happened with the full blessing of the church.

How the Church of England has shifted on divorce, from Henry VIII to Meghan Markle

Still, Charles’s admitted adultery (with Camilla) during his marriage to Princess Diana before their divorce in 1996 doesn’t sit right with some British people.

“Hard to celebrate a man who has been an adulterer and has well-known if arcane religious views,” said Bates, the former Guardian correspondent. “If the monarchy stumbles, where does that leave the established church?”

In some ways, Charles’s brand of faith — with greater focus on spirituality than dogma — puts him more in line with the British public.

Bradley said a small movement within the church already wants to see it formally uncoupled from the monarchy and the government. In a country with so many faiths, and so many people who don’t identify with any faith, Bradley said critics of the church wonder “if it can really still claim to be the church of the nation.”

“He has given us a lot of confidence,” said Zara Mohammed, head of the Muslim Council of Britain, the largest group representing the U.K.’s approximately 3 million Muslims. “We regard him as an admirer of Islam and a friend of British Muslims. It’s brilliant to see how he grasps how the U.K. has changed. He sees a more holistic picture and the power of all faiths and diverse communities working together.”

While it’s unlikely that any change in monarch would bring people back into the Church of England, Charles could be a more relatable “Defender of the Faith” for some church members.

“He represents those people who perhaps don’t have a vibrant faith, but have a sense that there is loving God,” said Andi Britt, 58. Britt is a human resources executive for IBM in London, who came with his wife, Jane, on Sunday morning to place flowers in the queen’s honor at Buckingham Palace.

“He represents a faith and a God who welcomes people, regardless of how close they feel,” said Britt, who described himself as a “committed Christian” and Church of England member. “I think he represents many people who are just not as sure, or who don’t have such strong convictions — people of faith, different faiths, or no faith.”

Boorstein reported from Washington.