In 1054, Pope Leo IX excommunicated the patriarch of Constantinople, an occasion that would go down in history as the beginning of the "Great Schism" between the Roman Catholic and Eastern Orthodox churches.
With the 1,000-year anniversary of the split just a few short decades away, Pope Francis and Patriarch Kirill of Moscow and All Russia will meet Friday in Havana as they attempt to bury the hatchet.
The tension between the churches of Rome and Moscow is part of the larger division between Catholicism and Orthodoxy, a split that dates to the earliest days of Christendom, when Christianity was first spreading through the Roman Empire.
"The eastern part of the Roman Empire spoke mostly Greek and the western part spoke mostly Latin," notes Justo L. Gonzalez, the author of several books on the history of Christianity. "So very soon already, in the second century, you begin getting different emphases between the Eastern church and the Western church."
Those differences widened after the collapse of the Roman Empire in the fifth century and the rise of the Byzantine Empire. Latin died out as a popular language, surviving only as the language of liturgy. But Greek flourished.
In the East, centered around Antioch and Constantinople in what is now Turkey, Greek priests spoke directly to their worshipers. In the years that followed, the Christian church in the eastern part of the old Roman Empire was tied closely to the popular culture.
In the western parts, the Christian church remained firmly tied to Rome, with a clergy answerable to the pope, also known as the Bishop of Rome.
As the Eastern or Orthodox version of Christianity spread across the Balkans and beyond to Russia, its distinguishing features were maintained: Each nation had its own Orthodox church with its own patriarch, associated with the national language.
"It's a matter of each of the two churches being very deeply enculturated in its own setting and having difficulty understanding the other," says Gonzalez.
The division extended beyond language and political loyalty. Roman Catholicism and Eastern Orthodoxy diverged even in how they practiced Christianity.
Catholic clergy are celibate, while Eastern or Orthodox clergy are allowed to marry. The churches also differ in the practice of the eucharist or communion ritual. The Roman Catholic Church uses unleavened bread for the communion host, while the Orthodox churches use leavened bread.
"There were even disagreements on a real deep theological level about the Trinity, concerning the relation between the Father, the Son and the Holy Spirit," says Brett Whalen, a professor of medieval studies at the University of North Carolina.
Such differences did not bring a complete break between Eastern and Western Christianity, however, until bishops in the East began looking almost exclusively to their own patriarchs for guidance — largely ignoring the pope.
"It really isn't until the 11th century that you see the pope really starting to emphasize and assert authority as the head of the church," Whalen says. "This contributes to generating some new antagonisms with the [Orthodox] church. "
That led to Pope Leo IX's excommunication of the patriarch of Constantinople — who returned the favor and in turn excommunicated the pope.
Four centuries later, with the fall of Constantinople to the invading Ottoman armies in 1453, East-West enmity deepened. Forces loyal to Byzantium felt the Papal States around Rome had given them insufficient military support. The centuries that followed brought a rise in the East of anti-western sentiment, especially in Russia.
Only in the modern era has there been much formal communication between Roman Catholic and Orthodox leaders. And the Russian church has largely stayed out of the conversation.
"There have been international commissions between the Roman Catholics and the Orthodox that have met repeatedly on a number of issues," says A. Gregg Roeber, a professor of early church history at Penn State University. "And it's precisely those dialogues that have troubled some elements in the Russian church."
While Friday's meeting in Havana is the first between a pope and a Russian patriarch, Francis invited the patriarch of Constantinople to his installation as pope in 2013. The patriarch accepted the invitation. Though small, the Patriarchate of Constantinople is generally seen as the cradle of Orthodoxy and therefore has historical and symbolic importance.
"That [2013 occasion] was hugely historic," Roeber says, "because that had never happened. I would not quite place the meeting in Havana on quite that level of significance." But, he says, it matters because it may "indicate a Russian willingness to try to integrate themselves more into these ongoing discussions between Rome and the rest of Orthodoxy."
Gonzalez, who is Cuban-American, believes the choice of Havana as a meeting site was no accident. "This meeting could not have happened in Western Europe," he says. "It could not have happened in Russia. It could not have happened even elsewhere in Latin America, which is mostly seen as Catholic. Cuba is a relatively neutral place."
Whether the encounter turns out to be truly historic, however, depends on what is actually achieved, Gonzalez says. "Something can only be considered historic after it becomes history," he says.