Leo XIV on Sunday celebrated an outdoor Mass formally inaugurating him as the 267th pope of the Roman Catholic Church, telling world leaders, the assembled hierarchy of his church and more than 100,000 faithful gathered in St. Peter’s Square that the church needed a new unity that recognized its diversity.

In his homily, the new pontiff sought to bridge the divisions in the church between those who want to engage with the modern world by pushing ahead with the inclusive approach of Francis, his predecessor, and more conservative elements who favor a return to an emphasis on the church’s traditions.

The cardinals tasked with electing a pope, he said, had sought a “shepherd capable of preserving the rich heritage of the Christian faith and at the same time, looking to the future, in order to confront the questions, concerns and challenges of today’s world.”

Leo, an American who has Peruvian citizenship, also said in his homily, that “I was chosen, without any merit of my own, and now, with fear and trembling, I come to you as a brother.” He added that he wanted “us all to be united in one family.”

The morning also marked Pope Leo’s first steps into a global role. He met on Sunday with President Volodymyr Zelensky of Ukraine, which he said, “awaits negotiations for a fair and lasting peace,” called for peace in Myanmar and urged the world not to forget those “reduced to hunger.”

Vice President JD Vance, who had a contentious meeting in the White House earlier this year with Mr. Zelensky, was also in attendance and shook hands briefly with the Ukrainian leader before the Mass began. After the Mass, Mr. Vance, who the White House called “the first Catholic convert,” to serve in the position, briefly shook the pope’s hand on a long receiving line of dignitaries and royals.