Francis will also minister to the countries' tiny Catholic communities and meet with migrants, highlighting other papal priorities during the three-day visit to two of Europe's poorest countries, where unemployment and brain drain are posing threats to the future.
It's the first trip by a pope to North Macedonia, and will likely boost the landlocked country as it embarks on a new relationship with Europe following its name change that ended a decades-long dispute with Greece.
__ CATHOLIC-ORTHODOX TIES The most sensitive part of Francis' trip will be his meetings with Orthodox leaders, part of the Vatican's longstanding effort to heal the 1,000-year-old schism that divided Christianity.
The conservative Bulgarian Orthodox Church doesn't participate in official Catholic-Orthodox dialogue and its Holy Synod governing body has made clear that it will not take part in any joint services or prayers with the pope.
Francis is expected to meet with its leader, Patriarch Neofit, and be accompanied by a Synod member into the Orthodox cathedral. But Francis will pray there alone. "We would like to emphasize that any form of shared liturgical or prayer service, as well as wearing of liturgical garments, is unacceptable to us as the holy canons do not allow this," the Holy Synod said ahead of Francis' trip. It said no blessing was given for clerics to participate in other papal events and said that ban extended to the Patriarchal Choir.
However, a Bulgarian Orthodox children's choir is expected to sing for the pope at a peace meeting, said Vatican spokesman Alessandro Gisotti. Francis has a different tightrope to walk in North Macedonia, where the Macedonian Orthodox Church declared its independence, or autocephaly, in 1967 but has not been recognized by other Orthodox churches. Serbian, Bulgarian and Greek Orthodox leaders, who all seek dominance over the country's Orthodox faithful, consider the Macedonian Orthodox Church schismatic.
Macedonian Orthodox leaders are hoping the country's name change will improve its chances of recognition before the Ecumenical Patriarch of Constantinople, the first among equals in the Orthodox world.
"Ties between the Macedonian Orthodox Church and Vatican are respectful . but more distant now because the Macedonia church's goal is to be recognized by others," said religious analyst Branko Gjorgjevski.
THE CATHOLIC COMMUNITY
Catholics represent less than 1 percent of the populations of each country, but the communities are nevertheless vibrant and have strong connections to two of the towering figures of the 20th-century Catholic church who greatly influenced Francis' ministry and were recently canonized by him.
St. John XXIII, the former Angelo Roncalli, was the Vatican envoy to Bulgaria from 1925-1934, and is so popular that the "Bulgarian pope" has a street in Sofia named after him.
In the Catholic stronghold of Rakovsky, Francis will meet with the Catholic community in the St. Michael Archangel Church, which Roncalli helped rebuild after a devastating 1928 earthquake.
In North Macedonia, Francis will pay homage to Mother Teresa, born Anjeze Gonxhe Bojaxhiu in Skopje in 1910. Francis shares a strong affinity with the nun's commitment to the poorest and most marginal.
He will pray at the Mother Teresa memorial in Skopje, built on the ruins of a church where she was baptized that was destroyed in a 1963 earthquake, and lay the cornerstone of a new sanctuary in her honor.
Bulgaria, which joined the European Union in 2007, has the lowest monthly average salary in the bloc and the smallest average monthly pension.
The World Bank has warned that Bulgaria needs to grow at 4% over the next 25 years to catch up with the rest of the EU. But widespread corruption and graft are draining the economy and pushing foreign investments away.
Bulgaria, with the bloc's highest mortality rate and the one of the lowest birth rates, is also the world's fastest shrinking nation, with its current population of 7 million expected to dwindle to 5.4 million by 2050 and 3.9 million by the end of the century. Tens of thousands of workers also leave the country annually in search of new opportunities.
North Macedonia is also experiencing a brain drain, with some 600,000 people leaving in the past 15 years, thanks in large part to its 20.7% unemployment rate. More than a fifth of North Macedonia's population of 2.1 million live below the poverty line.
Francis will meet with poor people in Skopje who are being served by Mother Teresa's sisters, and will likely refer to social inequalities in his political speeches in each country. North Macedonian leaders expect he will strongly encourage the country's efforts to join NATO and the EU now that the conflict with Greece over the Macedonian name has been resolved.
THE BALKAN MIGRATION ROUTE
Francis is expected to visit a refugee center housing mostly Syrian and Iraqi refugees in Bulgaria, which has taken a tough stance against mass migration to Europe by sealing off its border to Turkey with a barbed-wire fence to prevent migrants from entering.
The pope has long sought to show solidarity with asylum-seekers seeking a better life in Europe, and has urged governments to build bridges, not walls, and do what they can to welcome and integrate migrants.
The message, though, has fallen largely on deaf ears in much of Europe.
THE "BULGARIAN CONNECTION"
The first and last time a pope visited Bulgaria was St. John Paul II in 2002, a visit best remembered for his definitive dismissal of a so-called "Bulgarian connection" to the 1981 attempt on his life.
During the May 24, 2002, meeting with then-President Georgi Parvanov, John Paul said he "never believed in the so-called Bulgarian connection because of my great esteem and respect for the Bulgarian people."
It marked the first time the Polish-born pope had publicly quashed lingering suspicions that Bulgarian secret agents were behind Turkish gunman Mehmet Ali Agca's attack, which seriously injured the pope.
Agca served 19 years in an Italian jail and was pardoned by Italy's president at the Vatican's request in 2000.
Toshkov reported from Sofia, Bulgaria, and Testorides from Skopje, North Macedonia.