Pope Francis arrived in Seoul on Thursday looking to fuel a new era of Catholic growth in Asia -- a mission fraught with complex political challenges but huge potential rewards.
Regional tensions were underlined just minutes before the pontiff stepped off his plane, with North Korea firing three short-range projectiles into the sea off its east coast.
Francis is expected to send a message of peace to Pyongyang when he holds a special inter-Korean "reconciliation" mass in Seoul next week.
North Korea had been invited to send a group of Catholics to attend the event but declined, citing anger at upcoming South Korea-US military drills.
The North pays lip-service to the freedom of worship but maintains the tightest controls over religious activity and treats unsanctioned acts of devotion as criminal.
The pope's five-day visit to South Korea is recognition for one of Asia's fastest-growing, most devoted and most influential Roman Catholic communities.
Around one million people are expected to descend on downturn Seoul for an open-air mass on Saturday that will see Francis beatify 124 martyrs persecuted during the early days of the Korean Catholic Church in the 18th and 19th centuries.
But the real goal is longer-term and much wider-ranging.
The pope will bring a message about the "future of Asia", and will use his trip to "speak to all the countries on the continent", the Vatican's number two, Secretary of State Cardinal Pietro Parolin, said in a television interview.
That message will include warnings about income inequality and the dehumanising potential of rapid economic growth in countries like South Korea and China.
The last papal visit to Asia was by John Paul II to India in 1999, a glaring 15-year gap for a region where the Church is making some spectacular gains but where Catholics still only account for 3.2 percent of the population.
Message to China
The pope's flight to South Korea took him over China -- potentially the greatest prize of all, but also the hardest to claim. Beijing maintains a state-controlled Catholic Church, which rejects the Vatican's authority.
China "is a great cultural challenge, very great," Francis said in a recent interview with the Italian daily Il Messaggero.
The flight path offered Francis a rare opportunity to speak directly to Chinese President Xi Jinping, since the pope always sends a message to leaders of those countries he travels over.
"Upon entering Chinese air space, I extend best wishes to your excellency and your fellow citizens, and I invoke the divine blessings of peace and wellbeing on the nation," the message said.
China's Communist regime broke ties with the Vatican in 1951, and they remain firmly at odds over which side has the authority to ordain priests.
Francis was welcomed at the airport by President Park Geun-Hye with the two scheduled to hold formal talks later in the day.
The reception committee also included several North Korean defectors, and four relatives of victims of April's ferry disaster in which 300 people -- mostly schoolchildren -- were killed.
Meeting Asian youth
Francis will have a chance to address believers across the region on Friday when he meets several thousand young Catholics gathered in South Korea for Asian Youth Day.
"The pope's presence is a powerful symbol of the Vatican's recognition that it is in Asia and sub-Saharan Africa that the Church is growing most prominently," said Lionel Jensen, an expert on religion in Asia at the University of Notre Dame.
The pope has already announced visits to the Philippines and Sri Lanka in 2015, underscoring, Jensen said, "the new and very significant orientation toward Asia".
It is the first papal visit for 25 years to South Korea, which provides a model that the Vatican can only hope other Asian countries might follow.
The economic "miracle" that turned it from a war-devastated backwater to an export powerhouse and Asia's fourth largest economy in a little over five decades, was accompanied by an equally dramatic boom in Christianity.
Christians now comprise the largest religious bloc. While Protestants make up the majority, Catholics are growing faster -- accounting for more than 10 percent of the 50 million population, with tens of thousands of new baptisms every year.