Pope Francis starts a trip on Sunday to Bulgaria and North Macedonia where he will have to tread carefully because of sensitive relations with the dominant Eastern Orthodox Church in the two Balkan countries where Catholics are a tiny minority.
Bulgaria, a country of 7.1 million people, is home to just 58,000 Catholics, while North Macedonia, with a population of 2 million, has just 15,000 Catholics, less than some single neighbourhood parishes in Rome.
One purpose of the three-day trip is to improve relations with the Orthodox churches as part of the Vatican's push for eventual unity between the Eastern and Western branches of Christianity that split in 1054.
But that task is delicate because Orthodox churches in both countries are caught up in their own internal conflicts, which have spilled over into official relations with Catholics.
Bulgarian Orthodox leaders have ordered clergy not to take part in prayers or services with the pope, saying its laws do not permit it. But the pope will meet Orthodox Patriarch Neophyte and visit an Orthodox cathedral in Sofia.
"Receiving the pope but not praying with him is a contradiction in terms," said Tamara Grdzelidze, professor of Ecumenical Theology and visiting fellow at St. Michael's College at the University of Toronto. She suggested that the choice was due to internal disputes among Bulgarians.
A statement from the Bulgarian Orthodox Church last month explaining its position emphasised that the invitation for the pope's visit was made by state authorities, suggesting it had been given only a secondary role in the planning.
Bulgaria's Orthodox community is one of the most hardline in relations with the Catholic Church.
It is the only Orthodox community that has boycotted the most recent meetings of the official Orthodox-Catholic dialogue and also boycotted the 2016 Pan-Orthodox Council, citing differences on preparatory texts.
The Orthodox world considers North Macedonia's Church to be in a state of schism since it declared itself autocephalous, or independent, from the Serbian Orthodox Church.
Apparently in an effort not to upset other Orthodox Churches, the pope will not be meeting privately with North Macedonian Orthodox Primate Stephen.
It will be only the second visit by a pope to Bulgaria - Pope John Paul visited in 2002.
It is the first by a pope to North Macedonia and comes just three months after its name was changed from Macedonia, ending a decades-old dispute with Greece and opening the way for the ex-Yugoslav republic to join the European Union and NATO.
"It's a big political gesture on the part of the pope towards countries that have struggled to open themselves up both religiously and politically after the fall of communism and the Socialist bloc," Grdzelidze told Reuters.
"It could also be an encouragement for the local Catholic churches, despite their size, to be more active in contributing to public life and introducing Western values while not being in contrast to the Orthodox," said Grdzelidze, a former Georgian ambassador to the Vatican.
Francis is most eagerly awaited in Rakovski, Bulgaria's largest predominantly Roman Catholic town.
"It is a great joy, a great spiritual experience, a feast of faith for the whole community here in Rakovski as well as for the whole country," said Sister Elka Staneva, a nun who has been preparing local children to receive their first communion from the pope.
He will spend Tuesday in the North Macedonian capital of Skopje, where the late Mother Teresa was born Anjeze Gonxhe Bojaxhiu to Albanian parents in 1910 when it was still part of the Ottoman Empire.
Known as the "saint of the gutters" for her work among the poor in India, she died in 1997 and was officially made a saint by Pope Francis in 2016. He is due to visit her memorial and meet poor people helped by the order of nuns founded by the Nobel Peace Prize laureate.