HAVANA — In brokering the historic thaw between Cuba and the United States, Pope Francis stepped squarely into the thorny realm of geopolitics, sending letters to the presidents of both nations, playing host to secret meetings in the halls of the Vatican and nudging the Cold War enemies to put a half-century of vitriol and mistrust behind them.

But as he arrived in Havana on Saturday, the first stop of a nine-day papal trip to Cuba and the United States, Francis faced a new challenge altogether: Having helped open up Cuba to the world, the first Latin American pope must now try to fully open up Cuba to the Roman Catholic Church. “It is an occasion to ask for more openness,” said the Rev. Jorge Cela, who oversaw the Jesuit religious order in Cuba from 2010 to 2012. “The relationship is not easy.”

Francis was greeted at the airport by President Raúl Castro and children who welcomed him with flowers.