When the pope and the patriarch come together for the first time today, one of Christianity's most enduring divisions could edge closer to becoming ancient history.
The Roman Catholic and Orthodox churches have been at odds for over 1,000 years. Friday's meeting in Havana — where Pope Francis and Patriarch Kirill will sign a joint accord — could be a critical step towards helping heal the rift.
But while the Vatican billed the religious rendezvous as an "important stage" and sign of hope, the event could be more about symbolism than substance.
A MILLENIUM OF DISCORD
It took two years for the Holy See and the Patriarchate of Moscow to hash out the terms for the meeting — not long in the grand scheme of things.
The Western and Eastern Christian churches split over persistent theological disputes in 1054 — and formally separated in 1438.
The Eastern faction later became known as Orthodox Church — which now has 15 separate and equal congregations, Russian Orthodox being the largest.
Today's Russian Orthodox church refuses to recognize papal authority and has accused the Vatican of attempting to convert its followers.
"There was a tension between the two churches particularly from the Orthodox side," said Dr. Erica Hunter, head of Religions and Philosophies at SOAS University in London. "It's a division between cultures and attitudes towards the West in Russia."
The tensions and turf wars for religious dominance that punctuated the last 10 centuries have persisted in the modern era, creating an atmosphere of mistrust between the two churches.
Pope John Paul II — credited with an important role in ending the Cold War — invited his counterpart to meet more than once. But for the Orthodox Church, any appearance of submitting to Rome's authority was a deal-breaker.
"There are plenty of people among Russia's believers and clergy who are historically quite hostile to Catholicism… seeing it as a threat and part of the West that's looking to conquer Russia," explained Sergei Filatov, a religion expert at the Russian Academy of Sciences in Moscow.
AN ASSYMETRIC ARGUMENT
There are more than 1.2 billion Catholics around the world. That number dwarfs the overall Orthodox congregation of 225 million — which is divided between the 15 separate and equal eastern churches. The Moscow Patriarchate oversees the largest flock of about 160 million people.
Not only is it larger, but the Catholic Church has its own state: the Vatican. The Moscow Patriarchate, meanwhile, is closely aligned with the Russian government.
"Kirill is closely linked to Putin and Russian state power, while Francis is a genuinely independent operator," said NBC's Vatican expert George Weigel.
ARE THEY REALLY THAT DIFFERENT?
On many major theological issues Catholics and Russian Orthodox Christians remain closely aligned. But the issues that divide them run deep.
The central theological divide dates back to the eighth century and is based in differing philosophical interpretations of the Holy Trinity — the relationship between the Father, Son and Holy Spirit in Christianity.
The Orthodox Church also does not believe in purgatory, which Catholics believe precedes heaven.
There's the key political distinction — the Orthodox Church totally rejects papal authority — and differences on social issues. The Orthodox Church is more hardline on homosexuality, while Pope Francis famously said: "Who am I to judge?"
The question of clerical marriage is also treated differently by the Russian Church, in which parish-level priests are permitted to be ordained as married men.
Despite these issues, there is a lot less disagreement than there used to be.
Both churches have found themselves on the same side of the barricades, defending traditional religious values from rising secularism.
"Despite their differences, the Russian Orthodox church sees its closest ally in Catholicism now," said Sergei Filatov.
Experts say Friday's rapprochement comes down to personality — these two spiritual bosses are the right leaders at the right time.
"You need men with caliber, with vision, with understanding," said Hunter.
Francis — twice nominated for the Nobel Peace Prize — has been a champion of interfaith dialogue since his election three years ago.
Kirill, even before he took the Orthodox church's top job, showed unprecedented openness towards the Vatican as its chief diplomat.
World politics also has added urgency to the cause for Christian unity.
"Both of them are very aware of the plight of Christians in the Middle East," said Hunter. "Ecumenism and solidarity are important to the beleaguered Christians."
DOES IT MATTER?
While there's huge symbolic significance to the pope and patriarch's meeting, experts do not anticipate any major progress on the issues that have stood between their two churches for many centuries.
"It won't be huge headline leaps," Hunter said. "But all those steps to foster dialogue and to foster charity between two churches, and understanding, that takes a lot of hard, slow groundwork."
It's important to remember that while Kirill represents the largest flock of Orthodox, he doesn't speak for the whole Eastern church.
Still, that's not to say that overall progress isn't possible.
"They would probably not make any major agreements at the meeting, but the very fact of the meeting is a major breakthrough," Filatov said. "What happens now could start some bigger things, bigger collaboration in the future. And worst case, they just meet, and that's it."