It’s not hard to guess what topics Pope Francis and Prince Charles would like to talk about today: they have both issued heartfelt appeals to protect the environment and persecuted Christians, and both have recently expressed strong reservations about the rise of scapegoating populism.

But what if they don’t hover politely over areas they already agree on, and instead dig into an area they both care about, but on which they might not always see eye-to-eye? What if Charles seeks the pope’s advice on a question he has long been troubled by?

What if, in fact, when they meet this afternoon in the Vatican, they discussed the place of religion in politics — and specifically, Charles’s future constitutional role as Defender of the Faith?

The British, of course, don’t have a written constitution, but a centuries-old thicket of laws and norms regulating its different powers. Among these is the monarch’s role as Supreme Governor of the Church of England, and the title — bestowed, ironically, on Henry VIII by the then-pope — as Defender of the Faith.

That faith, of course, is the Protestant faith “by law established” — the Church of England, which since 1544 the monarch vows to defend in his or her coronation oath.

The pope dislikes established religion, believing, in line with Vatican II, that faiths are corrupted by being promoted by the state.

But he would be very supportive of Charles’s attempt to place his own role at the service of the faiths.

How, Holy Father — the heir to the throne might ask him — should I exercise my future role in a way that befits modern, pluralistic, multi-faith Britain? How do I protect and promote the Christian, Protestant faith by law established, yet in ways that build bridges?

Years ago, Charles sparked off an interesting debate when he referred to his desire to be a Defender not of the Faith, but of faith in general, or even “faiths.”

He wasn’t, he made clear in 2015, proposing to change the wording of the coronation oath, but asserting  — as did the Queen in 2012 — that the Church of England’s purpose “is not to defend Anglicanism to the exclusion of other religions,” but rather the free practice of all faiths.

Charles is an Anglican, but not, like his mother, a devotedly Protestant one. Where the Queen is firmly rooted in the low-Church Anglican tradition of the Bible and Book of Common Prayer (yet is very fond of popes and cardinals), Charles’s religiosity ranges both deep and wide.

He was for years under the strong influence of South African writer Laurens Van der Post, whose fascination with spirituality in the different world traditions rubbed off on the young Charles, who is passionate about his interfaith work. He has given highly regarded speeches to both Jews and Muslims, stressing their historic bonds with Christianity.

Yet he is not as eclectic as he is sometimes portrayed. While remaining a faithful Anglican, he identifies most closely with the Orthodox monastic tradition of his father’s family. His Russian great-aunts Alexandra and Elizabeth, killed by the Bolsheviks, converted to Orthodoxy; his grandmother was an Orthodox nun; and his father, Prince Philip, maintained links with the Greek Church even after converting to Anglicanism to marry the then Princess Elizabeth.

Charles is so close to Greek Orthodoxy, according to reports put out by monks but unconfirmed by him, that he has made a “spiritual commitment” to it while on Mount Athos.

The prince has been a regular visitor there, and there are Byzantine icons in the chapel at Highgrove, his country retreat, where he has been visited by Ephraim, abbot of the ninth-century Vatopedi monastery on Mount Athos, to whom he turned following the 1997 death of Diana, Princess of Wales.

His involvement in eastern Christianity explains why he cares so much about the destruction of the Christian communities of Iraq and Syria, which last Christmas he likened to the Nazi extermination of the Jews.

Charles has become a keen supporter of the Catholic charity Aid to the Church in Need, making generous donations to support its work, hosting refugees at his homes in Scotland and London, and speaks frequently of the plight of migrants.

Pope Francis will know all of this, and will encourage him in his role as bridge-builder and advocate, and promoter of the religious voice in the public sphere.

Perhaps the pope will want to support Charles’s embrace of a more active “interreligious dialogue” role as king, in which the head of state regularly brings together the leaders of different faiths to develop common responses to social challenges.

It is a role better suited to the monarch than to the government of the day, which tends to be ham-fistedly bureaucratic in its attempts to forge “community cohesion.” Governments tend to reduce faiths to sub-cultures, treating them as private organizations with values, like charities.

(In his La Croix interview last year, Francis criticized this Enlightenment-shaped tendency to consider “religions as sub-cultures rather than as fully-fledged cultures in their own right.”)

Charles, who is far more religiously and theologically literate than most of Britain’s politicians, would not make that mistake. He knows that nurturing the voice of faiths in public life is essentially an ecological endeavor: without the bonds of trust and spirit of service underpinning civil society that religion supplies, those bonds will be eroded over time by the state and the market.

The Queen has long been a model of servant-leader, Britain’s foremost volunteer, embodying the spirit of Christian service that underpins the best in British culture.

Charles could take that role and develop it, helping the faiths to find a common voice in technocratic, liberal Britain.

If Brexit is, in part, a re-run of the Reformation — Britain severing its ties with the European communion of Catholic faith — then a king committed to bolstering the faith ties that a religiously diverse Britain shares with the world would surely help to counter its isolating effects.

Here, then, is a mandate for the future King. And maybe this afternoon’s meeting with the religious bridge-builder par excellence will help him forge it.