ROME – When Pope Francis welcomes President Donald Trump in the Vatican on Wednesday, the eyes of the Western world – and probably beyond – will be on Rome, looking for signs of either agreement or clash between the heads of the world’s biggest temporal superpower and the world’s biggest spiritual superpower.

Few people from the United States’ side are more privy to what transpires in such a meeting than former ambassadors to the Vatican. For this reason, Crux spoke with the last five to hold the post, who gave a preview of what to expect, and shared their hypothetical advice to Trump.

The list includes Ken Hackett and Miguel H. Diaz, who served under Obama, and James Nicholson, Francis Rooney and Mary Ann Glendon, who served under Bush.

In keeping with a tradition that began in 1984, when President Ronald Reagan appointed a Catholic ambassador, every ambassador since has been a Catholic.

On Friday, the White House confirmed that Callista Gingrich, the wife of former House Speaker Newt Gingrich, will be Trump’s nominee for the post. However, she still has to be confirmed by the Senate, so it may be some time yet before she’s actually on the job.

This won’t be the first time a president meets a pope with no ambassador in town. The last time was in July 2009, when President Barack Obama met Pope Benedict XVI, while the Senate didn’t confirm Diaz until August.

The meeting gets a general green light

Nicholson, who served under Bush and helped organize two meetings between the president and Pope John Paul II, described the upcoming encounter as “a very important visit, and I’m really pleased that it’s happening.”

He believes Trump and the pope “ought to be acquainted and get to know each other.

“World problems aren’t solvable solely by material and military means,” he said. “It will take moral transformations for this world to live peacefully, which is why it’s important for the world’s spiritual superpower to meet with its most important temporal power.”

Hackett, who served under Obama’s administration, and was the last U.S. ambassador to the Vatican, agrees. He sees the meeting as an opportunity for “Trump to appreciate the personality and the concerns of the Holy Father, and for the Holy Father to take a measure of President Trump.”

All the ambassadors believe there’re many areas in which this summit could be fruitful, and all highlighted the importance of the U.S./Vatican relationship.

Rooney, who served during Bush’s second term, said he’d like to see “some of the same things we accomplished with George Bush” come out of it.

He spoke about the places in the world where the two nations have their diplomatic platforms aligned. Working together can help advance “our common goals of religious freedom, freedom of the press, human dignity, and promoting the natural rights of man.”

Diaz, appointed by Obama in 2009, a few months after his visit to Benedict, also acknowledged the possibilities.

“Cooperation on a wide variety of issues can be very fruitful and mutually rewarding for both sides,” he said. “In theory, the relationship between these two entities can produce important results if carried out strategically, and if both sides find a way to cooperate with one another.”

As Diaz said, both hold significant offices, with Trump being the “leader of the free world,” and the pope a head of a state but also the leader “of a pretty significant number of persons of faith, some 1.3 billion Catholics scattered throughout every corner of the world.”

Speaking about the meeting in particular, Glendon said she hoped for “strong statements on the steadily worsening persecution of Christians around the world, a topic on which the previous U.S. administration was shamefully silent.”

She also hopes there will be statements on threats to religious freedom in liberal democracies, where it is “currently in danger of being demoted from the status of a fundamental right to just one of many competing interests—one that can all too easily be trumped by other rights, claims and interests.”

Glendon also noted that for Trump, the meeting has particular significance because he received a substantial number of Catholic votes.

Looking at the broader picture, the ambassador highlighted that these meetings are always important because the Holy See, “tiny though it is,” can be “an influential partner on many common fronts.”

Both states are global actors, she said, and one of the shared concerns is improving the lives of people struggling with poverty, hunger and disease.

“The U.S. is the world’s largest donor of humanitarian aid, and the Holy See oversees the world’s largest network of hands-on providers of education, health care and relief,” she noted. “Another concern of huge interest to both is to strengthen the global consensus against terror, and against the use of religion as a pretext for violence.”

Briefing the president

Despite the Vatican being a state whose borders one can walk, literally, in about 40 minutes, all the ambassadors pointed out it’s hardly an insignificant one. There’s much more to the meeting than meets the eye, and its success, or failure, can be significant.

For this reason, briefing Trump – or any president for that matter – ahead of the meeting is a delicate task.

“Were he to call, I would suggest that he listen,” Hackett said. “He would learn quite a bit from listening to Pope Francis and the other people he will meet. And that gives him the beginning of a rapport.”

Such a rapport, Hackett said, would be more important than substantively getting “this point or that point” across.

Hackett had the job of briefing Obama for this meeting once, when he came to the Vatican in 2014 to meet with Pope Francis.

Nicholson, who briefed Bush twice, began by saying he’d advise Trump to “be as cordial as he can be,” and to expect the same from the pontiff.

He would also suggest the president to thank Francis for “his leadership, his exercise of his megaphone” on behalf of many, including “the poor and disenfranchised of the world, who are exactly the people Trump cares about the most.”

According to Nicholson, the president should also thank the pope for his leadership in “espousing the dignity of life,” religious freedom, for condemning trafficking in persons and 21st century slavery, his concern over international terrorism and his desire for peace and stability in the world.

Speaking about briefing the president not only for the meeting in general, but particularly on the areas in which the two disagree – from migrants and refugees to climate change – Rooney said that “you have to explain to the president what the pope’s positions have been, and hopefully they can either agree to disagree or find whatever common understanding can be obtained, the way we did with Iraq.”

When talking about Iraq, Rooney means the open opposition from John Paul II to the U.S.-led invasion back in 2003. The pope expressed his concerns regarding the war in many ways, including during his 2003 address to the diplomatic corps accredited to the Holy See.

“We finally got to the point with Iraq where it was, ‘Let’s make the best of a bad situation.’ Let’s focus on the Christians of Iraq, and the people getting killed every day, and stop talking about the fact that we disagreed wildly over whether to go in there,” the U.S. Representative for Florida’s 19th congressional district said.

Picking up on Francis’s recent comments about Trump, on his way back from Fatima, Portugal, when the pope said he wasn’t going to judge the president before the two actually meet, Rooney said this was the pope’s way of saying “’I’m looking forward, not backwards,’ the same way Pope Benedict talked about Iraq.”

Diaz argued that Francis’s vision, “whose signature teaching revolves around the denunciation of the globalization of human indifference” is “very much in tension with many of the policies of our sitting president.”

Trump, he said, should come to the meeting with “an open heart and an open mind,” and listen to Francis, whom Diaz defined as someone “who knows the art of diplomacy and human engagement.”

Diaz acknowledged the fact that both are outsiders, calling for institutional reform. Yet Francis’s way of doing it, “without ever ceasing to attend to the cry of the poor, oppressed and marginalized marks a huge difference between these two men.”

Glendon too, took a cue from Francis’s latest in-flight press conference to suggest the president look for “open doors,” areas of common concern: Persecuted Christians around the world, the erosion of religious freedom and conscience protection in the liberal democracies, the need to condemn the use of religion as a pretext for violence, the importance of protecting human life, the need to find creative ways to promote job creation, and to address human suffering and deprivation.

She also hopes the two “explore common ground in areas where the press tends to magnify differences—immigration policy, border security, stewardship of the environment, and concern for future generations.”

The belief that something fruitful can come out from the meeting is shared across the board, even if they don’t agree on the common issues, or the priorities. As Diaz put it, the two will speak their minds, and that’s not a bad thing.

“Part of the problem we’re in, whether at the local, national or international levels, is that we try to avoid controversial conversations,” Diaz said. “I don’t think that gets us anywhere.”

According to Diaz, a theologian whose ancestry is Cuban, the world can’t deal “with the challenges facing the human family with alternative facts.”

He’s less certain when it comes to finger pointing what issues will be discussed, and he puts this one on the administration. It’s hard to know, he said, “because there seems to be dissonance among the State Department, the White House, the president, and the rest of our government.”

This lack of coordinated communications, he said, makes it hard to “get mutual collaboration with the Vatican or anyone else.”

John L. Allen and Claire Giangravè contributed to this report.