Popemania! When Pope Francis made his first visit to the U.S. last month, more than just the nation's 67 million Catholics were paying attention.

• Author: Smith, Patricia
• Position: Religion

Standing in front of a packed chamber in the U.S. House of Representatives in Washington, Pope Francis challenged lawmakers--and, by extension, the world's most powerful nation--to respect the rights of immigrants, take bolder action to address climate change, and abolish the death penalty.

In other words, the spiritual leader of the world's 1.2 billion Catholics stepped into the middle of some of the most contentious issues in American politics.

On his first trip to the U.S., Pope Francis stopped last month in Washington, D.C., New York, and Philadelphia, and in each city hundreds of thousands of cheering people--and not just Catholics--lined the streets to get a glimpse of him.

The reception wasn't unusual. Since becoming pope two-and-a-half years ago, Francis has caused quite a stir inside and outside the Church, in both words and deeds. In the beginning, it was a series of small gestures: The former Cardinal Jorge Mario Bergoglio of Argentina chose the name Francis, after Francis of Assisi, a 13th-century saint known for his work with the poor. Instead of moving into the opulent palace in the Vatican where popes had lived for more than a century, Francis moved into a modest apartment.

At the Vatican, he replaced traditionalists with more moderate cardinals. He's stressed that serving the poor is central to the Church's identity and mission. And he's put less emphasis on divisive social issues than his predecessors. Asked by a reporter in 2013 about gays in the Church, Francis said nothing about homosexuality being a sin, which has long been the Church's position. Instead, Francis replied, "Who am I to judge?"

Francis hasn't changed any official Church doctrine: The Church still opposes divorce, abortion, same-sex marriage, and women becoming priests. But with his humble style and off-the-cuff remarks, he's reshaping how the Church is viewed.

"In the Catholic Church, symbols are substance," says John Carr, director of the Initiative on Catholic Social Thought and Public Life at Georgetown University in Washington. "Other popes have been remarkable men, but they've been surrounded by walls of bureaucracy. This is a pope who's taking down the walls."

Income Inequality & Climate Change

He's also pushing for change. Earlier this year, he helped broker the thaw in relations between Cuba and the U.S., frozen for more than half a century. He's called on Western countries to address the problem of income inequality. And he's issued an...