Pope Francis, the 266th pope and the leader of more than 1 billion Catholics across the world, will begin a six-day visit to the United States on Tuesday, which will include a historic address to Congress, the first time in the history of the country that a pope will address lawmakers.

His visit to the world’s most powerful nation is likely to be a time of reckoning. Francis in his papacy has displayed a keenness to engage with the most important issues of the day, from immigration to climate change, and his visit will be closely watched in the United States and around the world.

“The pope is coming as a pastor to the American Catholic Church. He is also coming as a prophet,” Thomas Reese, a priest and commentator for National Catholic Reporter, noted at a recent conference. “What a prophet does is comfort the afflicted and afflict the comfortable.”

While no one, not the leaders of the American Catholic Church nor the White House, is certain exactly what the pope will have to say, his itinerary is testament to his intention of doing exactly that. During his U.S. trip, the pope will have the ears of the powerful – be they members of Congress at the joint session Wednesday or world leaders at the U.N. General Assembly, which he will address Thursday. “He comes to speak truth to power” is how one commentator described Francis’ mission. His next stop after the unprecedented Congress joint session: a meeting with the homeless. Also on the agenda of the Argentinian pope, himself a son of migrants, is meeting with migrant families in the United States.

While Catholic leaders are going out of their way to emphasize that the pope is not coming to the United States as a politician, as a world leader he could not have chosen a more opportune time to visit. “The most important thing he is doing here is opening the General Assembly session on Sustainable Development Goals, one of the largest gatherings of world leaders,” said Anthony Annett, an climate change and sustainable development adviser at Columbia University’s Earth Institute. The pope’s U.N. address also comes as countries prepare for a landmark U.N. summit in Paris, where a new climate deal will be negotiated.

John Gehring, author of “The Francis Effect,” pointed to the pope’s decision to release an encyclical, one of the most authoritative forms of church teaching, centered on integral ecology and the need for environmental stewardship ahead of the climate negotiations as a sign that he wanted to steer the debate. One of the most notable features of the encyclical for environment activists was the pope’s reference to a scientific consensus on the “disturbing warming of the climatic system” and his call for urgent action to combat it.

Experts say that he has already expanded the contours of the debate on climate change by recasting it as a powerful moral concern in the encyclical. While Republicans tried to downplay its impact at the time of its release – for instance, Republican presidential candidate Jeb Bush, a Catholic convert, suggested that he did not take advice on economic policy from the church – even the staunchest climate deniers have seemed unwilling to take on the pope on the issue. It is expected that the pope will double down on this message while speaking in the United States, which is not only the largest economy in the world but also one of the world’s biggest carbon emitters.

And it is not just his message but his characteristic way of framing it in simple yet startling terms that has captured the hearts and minds of Catholics globally and even attracted admirers from other denominations. “His humility and simplicity” is an important part of his appeal, Annett said. Featuring the pope on its cover, Time magazine in 2013 described Francis as “The People’s Pope.” He is especially beloved among Latino Catholics, who are emerging as the face of the Catholic Church in the United States and have growing political clout.

Obama wants the visit to have ‘lasting value’

“He is the most compelling moral leader in the world, a spiritual troublemaker and a newsmaker,” Gehring said, “relentlessly focused on making it a church for the poor.” While reaching out to the poor and being a voice for the excluded has been an abiding theme of his papacy, it is his recent stand on climate change and trenchant critique of misguided development and environmental degradation that have drawn much attention in the media.

While politicians would be envious of his approval ratings, the pope is likely to leave many of them in an unenviable position when he addresses Congress next week. Even though almost 30 percent of the members of the current Congress are self-identified Catholics, according to a Pew Research Center survey, most Catholic Republicans would struggle to find common ground with their religious leader about the role of humans in climate change.

While most political observers expect U.S. lawmakers to be on their best behavior Wednesday, not everybody is ruling out some political theater. “When he comes in there and says, ‘Welcome the immigrant, care for the poor, protect the environment,’ the Democrats are going to go crazy, jumping up and down applauding,” Reese said, not without some amusement.

Outside the corridors of power, there are strong indications that the pope enjoys a unique position as a messenger on climate change issues. Conservative Catholics who attend Mass regularly respond more favorably to the pope’s message on climate change than to the same message from secular experts, a recent survey by YouGov, a digital market research firm, reported. He is also in the best position to get through to Republican voters, the survey found.

The White House in a press briefing yesterday downplayed the possible impact of the pope’s visit on domestic politics, saying that the pope “operates on a different plane.” However, they did note that the Obama administration and the Vatican share much “common ground.” Ben Rhodes, deputy national security adviser for strategic communications at the White House, revealed that President Obama in preparation for the pope’s visit directed his team to work toward ensuring “that this visit has lasting value.”

The visit, some experts believe, will have lasting value for those who are pushing for stronger and faster action on climate change.

“A pope who talks about the economics of inclusion and who insists that climate change is an urgent moral issue has the potential to help recalibrate a Catholic political narrative that in recent years has been narrowly defined,” Gehring said.

Climate change in this country has mostly been viewed as a debate over scientific evidence and economic calculations, but very little has been said about the moral aspects of it. “Elected members may easily dismiss secular environmental groups when it comes to climate change,” Annett said. “Brushing off the pope is another matter altogether.”