On his way to South Korea, Pope Francis will get a rare opportunity to directly address China's leadership as he flies over the country, whose communist government does not allow Catholics to recognise his authority.
The pope, who leaves Rome on Wednesday, always sends telegrams to the leaders of countries as he passes through their airspace. The routine messages rarely make news, but this time there is keen expectation for what the pope will say to China.
The fact that he is being allowed to cross Chinese airspace at all - Pope John Paul II had to skirt it in his tours of Asia - is seen as a positive, if small, step forward, in the often-fraught relations between the Vatican and China.
"This is a sign of detente, for sure," said Father Bernardo Cevellera, head of the Rome-based AsiaNews agency and a specialist in the Catholic Church in China. "But the real miracle would be if (Chinese President) Xi Jinping responds with his own telegram, and what he says".
The Vatican has had no formal relations with China since shortly after the Communist party took power in 1949. The Catholic Church in China is divided into two communities: an "official" Church known as the "Patriotic Association" answerable to the party, and an underground Church that swears allegiance only to the pope in Rome. The most contentious issue between them is which side gets to name bishops.
"The Holy See favours a respectful and constructive dialogue with authorities to find a solution to the problems that limit the complete practising of the faith by Catholics and to guarantee an atmosphere of real religious freedom," the Vatican's number two, Secretary of State Cardinal Pietro Parolin, told an Italian Catholic magazine.
The Vatican has been sending olive branches to China for decades, but a major stumbling block is the Holy See's continued recognition of Taiwan, which Beijing considers a renegade province.
Another irritant is what missionaries and aid groups have described as a crackdown by Chinese authorities on Christian charity groups near its border with North Korea in recent months. The sweep along the frontier is believed to be aimed at closing off support to North Koreans who flee persecution and poverty in their homeland and illegally enter China before going on to other nations, usually ending up in South Korea.
The two Koreas have been divided since the Korean war, which left millions of families separated. The South and the reclusive, communist North have been at a near-constant standoff since the 1953 armistice.
The Argentine pope leaves Rome for South Korea on Wednesday afternoon, and during the six-day trip he will hold a "Mass for Peace and Reconciliation" in the Myeong-dong cathedral in Seoul, the capital.
Officials of the Catholic Church in South Korea, which counts about 10% of the population of 50 million, said they had asked the North to send a delegation to a papal Mass, but the North said they could not "for various reasons".
North Korea's constitution guarantees freedom of religion provided it does not undermine the state, but outside of a small handful of state-controlled places of worship, no open religious activity is allowed. The regime has sentenced two foreign missionaries to hard labour and has jailed a US tourist for leaving a Bible in a toilet.
A United Nations report earlier this year cited estimates that between 200,000 and 400,000 of North Korea's 24 million people are Christians. The number is impossible to verify because most Christians cannot worship openly.
The pope' six-day visit, whose main purpose is for him to preside at a gathering of Asian Catholic youth, is the third international trip by Francis since his election in March 2013 and the first by a pontiff to Asia since 1999.
Francis is due to visit Asia again in January when he travels to Sri Lanka and the Philippines, underscoring the importance the Vatican attaches to an area with great potential for growth in Catholicism.
The Catholic Church in South Korea is a vibrant community, growing at a rate of about 100,000 new members a year, most of them adult converts.