According to the Catholic Church, a number of acts qualify as mortal sins: murder, apostasy, stealing from the poor, etc. But in the eyes of a Vatican official who spoke to Bloomberg News on Monday, there’s another kind of sin that qualifies as a “heinous act”: when a newspaper leaks a papal encyclical three days before it’s supposed to come out.

For more than a year, Pope Francis and his close advisors have been preparing this document, called Laudato Si, or Praised Be. The text focuses on environmental stewardship and, in particular, the effects of climate change on human life. The themes are directly in keeping with the rest of his papacy: When he was elected to the office, he told journalists he took the name “Francis” in honor of St. Francis of Assisi, who stood for the poor and for peace, and was a “man who loved and cared for creation ... in this moment we don’t have such a great relationship with the creator.” The official copy of the encyclical doesn’t come out until Thursday, but on Monday, the Italian magazine L’Espresso leaked an Italian version, which Church officials are calling a “draft.”

The Vatican has reacted so strongly to the leak because this encyclical is a very big deal within the Catholic world. It’s one of the most formal statements the pope can make about Catholic doctrine, and it’s the first of his papacy. (Last spring, he released another piece of writing on the topic of poverty, but it was a slightly less formal document called an apostolic exhortation.) Francis chose a theme that’s long been a focus for pontiffs: Benedict XVI is cited 21 times in the draft version of the text, and John Paul II is cited 22 times. But this is the first instance in which the environment has been a topic of an encyclical. “No pope has ever issued a statement [about the environment] on this level of document,” said Kevin Irwin, a priest and theologian who teaches at the Catholic University of America. “John Paul put it into a World Day of Peace message, but a World Day of Peace message is down the rung on the ladder of the hierarchy of Catholic documents. And Benedict gave a number of homilies and speeches on it, but never a document on this level.”

In the draft version of the document, the pope makes a strong case that humans are at fault for the degradation of the environment. “Numerous scientific studies indicate that the major part of global warming in recent decades is due to the high concentration of greenhouse gas … emitted above all because of human activity,” he writes. His thinking on the environment connects with other major themes of his papacy, including care for the poor and the importance of human life. In the draft, he writes that the heaviest impacts of climate change “will probably fall in the coming decades on developing countries. Many poor people live in areas particularly affected by phenomena related to heating, and their livelihoods strongly depend on natural reserves and so-called ecosystem services, such as agriculture, fisheries, and forestry.” He also discusses the effects on immigrants and refugees: Changing environmental conditions force them into a position of economic uncertainty in which they can’t sustain livelihoods, he writes.

What this encyclical is not is a love letter to Greenpeace—although Francis is embracing the idea of environmental stewardship, he's doing so as a Catholic theologian, not a liberal activist. Especially in the American press, the pope’s encyclical has often been discussed in terms of U.S. politics, where a significant minority of mostly Republican voters and legislators deny the existence of climate change. Earlier this month, Oklahoma Sentor James Inhofe said of the encyclical, “The pope ought to stay with his job, and we’ll stay with ours.” Rick Santorum, a Catholic former U.S. senator and presidential candidate, advised the pope to “[leave] science to the scientists and [focus] on what we’re good at, which is theology and morality.”

But, in fact, the topic of this encyclical is squarely in the pope’s wheelhouse. Francis links his call for environmental stewardship to the book of Genesis, and he repeatedly couches environmental degradation in theological language. “That human beings destroy the biological diversity in God's creation; that human beings compromise the integrity of the earth and contribute to climate change, stripping the earth of its natural forests or destroying its wetlands; that human beings pollute the water, soil, air; all these are sins,” he writes.

Although American Catholics are a sizable group, they’ve got nothing on the whole of Francis’s church: There are 1.2 billion Roman Catholics in the world, and nearly 40 percent of them live in South America, not North America. Sub-Saharan Africa is another area of rapid growth for the Church; demographers expect the number of Christians in the region to double by 2050 to nearly 1.1 billion, although some of those will be Protestants. Considering that Latin America and Africa are Francis’s two biggest “constituencies,” it’s no wonder that the environment is a point of pressing concern for the global Church: Climate change affects those who are poor and live in developing countries much more intensely than those who live in the developed world. Francis is coming out against climate change, yes. But he’s mostly continuing the focus of his entire papacy: speaking for the world’s poor.