Elizabeth Eaton is in a bit of a bind. Exactly 500 years ago on October 31, Martin Luther allegedly nailed his famous “95 Theses” to the door of a German church, decrying the Catholic Church’s abusive sale of indulgences, or reprieves from punishment for sins. The monk’s dramatic declaration set in motion years of theological sparring and bloody wars. It also led to the flowering of Protestantism and its many distinct denominational traditions, including the Evangelical Lutheran Church in America, over which Eaton presides as bishop.

The thing is, “that was not good for the church,” Eaton said, referring to the global body of Christian believers. The New Testament calls for followers of Jesus to be “completely one.” By Luther’s time, Christianity had been split from East to West as the Orthodox and Roman Catholic Churches parted ways. Within those traditions, dissidents had already formed a number of prominent sects. The Protestant Reformation catalyzed further breakdown.

“In the 16th century, we were killing each other over these issues,” Eaton said. Five centuries later, the ELCA and other churches around the world are marking Luther’s big moment. But “we are not celebrating the 500th anniversary of the Reformation,” Eaton said. “We are observing it.”

Efforts to create Christian unity, or ecumenism, often carry this note of historical sadness. Prejudices borne of past violence are difficult to overcome. For more than a century following Luther, Christian reformers and their political allies across Europe battled with the Catholic Church-aligned Holy Roman Empire. Catholics slaughtered Protestants; Protestants slaughtered Catholics; and both persecuted groups like the Anabaptists, who championed adult rather than infant baptism. Most traditions did not develop their distinctiveness by accident; many religious leaders staked their lives on their particular interpretation of the Bible. (“I cannot and will not recant anything, since to act against one’s conscience is neither safe nor right,” Luther famously told the Holy Roman Emperor Charles V in the face of excommunication and death.)

While relations among Christians are far more peaceful today than they were 500 years ago, the tension between theological particularity and yearning for universal fellowship is still just as complicated. As global Christianity evolves, the tension is likely to increase.

Especially over the last century or so, Christian groups have made significant attempts to repair the conflicts among them. In the mid-19th century, the Evangelical Alliance sought to unite Protestant groups to oppose child labor and poor factory working conditions, a unity they described as “a new thing in church history.” In 1910, a missionary conference in Edinburgh laid the groundwork for what later became the World Council of Churches, which united many Eastern Orthodox, Anglican, and mainline Protestant churches for the first time.

But until recently, the rifts of the Reformation were insurmountable. “The idea that Catholics and Protestants would get together to cooperate on anything is just almost unimaginable before the 1960s,” said Mark Noll, a historian at Notre Dame University. “In my lifetime, there has been a sea change in Protestant-Catholic relations, opening up an unimaginable array of cooperation.”

“We also look silly to the rest of the world, I think, if we’re all fighting with each other.”

A number of scholars I spoke with cited as a turning point the Second Vatican Council, which declared “restoration of unity among all Christians” one of its “principal concerns.” In 1999, the Lutheran World Federation and the Catholic Church even reached a consensus on the basics of the doctrine of justification—how people get to heaven—which had been a key dividing point during the 16th-century Reformation.

This sense of fellowship, if not full unity, has carried through to the Reformation festivities. Last October, Pope Francis met with Lutherans in Sweden to express longing “for this wound in the body of Christ to be healed,” hoping that one day, Lutherans and Catholics can once again share the celebration of the Eucharist, or the consecration of bread and wine as Jesus’s body and blood. Most remarkably, their joint declaration acknowledged that “the Reformation helped give greater centrality to sacred Scripture in the Church’s life”—a stunning sentiment for a pope to express in a country where Catholic convents were banned until the 1970s.

And yet, this atmosphere of optimistic dialogue papers over significant divisions. Lutherans don’t agree with the Catholic Church on the authority of the pope or the way their churches should be governed by bishops. They themselves are not unified: The Lutheran World Federation, which represented Lutherans at the conference in Sweden, doesn’t actually speak for all Lutheran denominations. Prominent groups with more than 1 million members, such as the Lutheran Church-Missouri Synod in the United States, remain separate from even those who share a denominational name.

“It’s kind of a scandal, and it’s also a point of sorrow, because it divides the family,” said Eaton, whose more liberal denomination, the ELCA, is part of the Lutheran World Federation. “We also look silly to the rest of the world, I think, if we’re all fighting with each other.” Yet Samuel Nafzger, a former LCMS leader, wrote in a 2009 paper that doctrinal differences matter—and they’re best resolved directly, “not by ignoring them or by agreeing to disagree.”

“Some Eastern Orthodox clergy … believe ecumenism is the work of the devil.”

Some within the Catholic Church are also skeptical of Pope Francis’s aggressive outreach to Christians of other traditions. “I think he’s tried to follow a cautious line in relationships with other Christian groups,” said Father Russell McDougall, the rector of Tantur Ecumenical Institute in Jerusalem, “because he realizes that if he gets too far ahead of his flock, he’ll lose them.” While the pope has worked on reaching out to Protestants, his ecumenical energy has more often been directed toward Eastern Orthodox churches, which deserve “special consideration” based on their close heritage with the Roman Catholic Church, according to Vatican II documents. There, too, some are skeptical. In Israel and elsewhere, for example, “some Eastern Orthodox clergy … believe ecumenism is the work of the devil,” said McDougall.

The goal of ecumenical dialogue can be hazy. Tantur was opened by Notre Dame in the 1970s in the wake of Vatican II to “assist the search for Christian unity and interchurch harmony.” And yet, McDougall said, some in the Catholic Church still think “ecumenism means, ‘Enter into dialogue with others to show them Catholicism.’” He believes that, more than ever before, “Christians now … recognize that our own narratives that we tell our congregations for catechistic purposes have been simplistic.” But living out that recognition can be rocky. “There still is a reluctance on the part of some Christians to pray with other Christians at all,” McDougall said. “The Eucharist is still a tender point … For the most part, Catholics don’t invite Protestants to communion in their churches, and the Orthodox don’t invite anyone.”

Even when leaders of different Christian traditions find theological agreement, “the churches don’t really know what to do with the results,” said Father Frans Bouwen, a member of a Catholic order called the Missionaries of Africa, or White Fathers, who run Saint Anne’s Basilica in Jerusalem. In his years overseeing a journal on ecumenism, Proche-Orient Chrétien, Bouwen has found that theologians and top Church leaders have more energy for ecumenical work than local religious leaders. “This is in some way disappointing,” he said. “The priests say, ‘We have done the work, but the churches do nothing.’”

Ecumenical efforts may find most difficulty in the parts of the world where Christianity is changing and growing most quickly. Bouwen said ecumenism is low on the list of priorities for missionaries who work on evangelization in Africa, where Catholicism has seen its greatest rate of expansion over the last couple of decades. Younger churches, including the Pentecostal and evangelical congregations that have seen similar success in Africa in recent years, are often underrepresented in ecumenical movements, Bouwen added: They don’t carry the same history of intra-Christian conflict and don’t necessarily view ecumenical problems in the same way as more established churches. Plus, he said, they’re busy evangelizing their particular brand of Christianity.

Meanwhile, Christian denominational families continue to fracture. The United Methodist Church, which claims 12 million members around the world, and the Anglican Communion, which claims 85 million, have both experienced bitter conflicts in recent years over LGBT issues. These divisions have been driven in part by the different orientations of relatively progressive American churches and their relatively conservative counterparts in Africa and other parts of the global South.

While these disagreements have led to painful splits—both denominations face the possibility of schism in the coming years—they also suggest that a growing number of Christians are organizing themselves based on ideological convictions, rather than a shared confessional tradition. “As a lot of denominational traditions are experiencing pressure and even fracture,” said Noll, “so also [is] interdenominational cooperation amongst like-minded people growing in leaps and bounds.”

Some of the ecumenical reformers of the 1970s and ’80s imagined they’d see full Christian communion in their lifetimes, Bouwen told me; he himself was part of that generation. Clearly, that has not come to pass. He’s a bit more jaded than he was as a student in Rome during the heady years of Vatican II; he’s spent too much time in Jerusalem, where, he said, “unity is a scandal.” Despite himself, though, he still holds out hope that the church can become whole again.

“Ecumenism is a microbe,” he said. “You either have it or you don’t have it. And if you have it, you can’t get rid of it.”