VATICAN CITY — Ban Ki-moon arrived at the Vatican with his own college of cardinals. Mr. Ban, the United Nations secretary general, had brought the leaders of all his major agencies to see Pope Francis, a show of organizational muscle and respect for a meeting between two global institutions that had sometimes shared a bumpy past but now had a mutual interest.

The agenda was poverty, and Francis inveighed against the “economy of exclusion” as he addressed Mr. Ban’s delegation at the Apostolic Palace. But in an informal meeting with Mr. Ban and his advisers, Francis shifted the discussion to the environment and how environmental degradation weighed heaviest on the poor.

“This is the pope of the poor,” said Robert Orr, who attended the May 2014 meeting as Mr. Ban’s special adviser on climate change and described the informal conversation with Francis. “The fact that he is making the link to the planet is really significant.”

On Thursday, Francis will release his first major teaching letter, known as an encyclical, on the theme of the environment and the poor. Given the pope’s widespread popularity, and his penchant for speaking out on major global issues, the encyclical is being treated as a milestone that could place the Roman Catholic Church at the forefront of a new coalition of religion and science.

Francis, the first pope from the developing world, clearly wants the document to have an impact: Its release comes during a year with three major international policy meetings, most notably a United Nations climate change conference in Paris in December. This month, the Vatican sent notifications to bishops around the world with instructions for spreading the pope’s environmental message to the more than one billion Catholics worldwide.

By wading into the environment debate, Francis is seeking to redefine a secular topic, one usually framed by scientific data, using theology and faith. And based on Francis’ prior comments, and those of influential cardinals, the encyclical is also likely to include an economic critique of how global capitalism, while helping lift millions out of poverty, has also exploited nature and created vast inequities.

“We clearly need a fundamental change of course, to protect the earth and its people — which in turn will allow us to dignify humanity,” Cardinal Peter Turkson of Ghana, who oversaw the drafting of the encyclical, said at a conference on climate change this spring at the Vatican.

Vatican officials say that the encyclical is a theological document, not a political one, and have refused to divulge the contents. But there is already much speculation about how Francis will comment on humans’ role in causing climate change, a link he has spoken about in the past. The Vatican’s scientific academy recently attributed climate change to “unsustainable consumption” and called it “a dominant moral and ethical issue for society.”