On Oct. 31, 1517, a German monk and professor named Martin Luther raised serious questions about how Catholicism operated. Christianity hasn't been the same since. But after almost 450 years of animosity between Catholics and Protestants, the last half-century has witnessed great strides in dialogue and reconciliation. A shared sense of that complicated history offers lessons for a country and world severely torn apart by political, social, national, and religious tribalism.

Some readers may remember the days when a Catholic and a Protestant could not get married in church. Some parents wouldn't even let their children play with other kids who practiced a different brand of Christianity than their own. Those days are over thanks to mutual overtures between Protestants, especially Lutherans, and Catholics. That reaching across the church aisle was fueled by a spirit of ecumenism found in a document from Catholicism's Vatican II in 1964 titled in Latin Unitatis Redintegratio—"restoration of unity." It invited all Christians to dwell more on their shared faith in Jesus as the resurrected Son of God rather than doting on what set them apart.

This was a huge change. At the start of the reformations, Luther was calling the papacy the Antichrist, but it wasn't long before other reformers who thought Luther was moving too slowly and cautiously labeled Luther himself the Antichrist, too. For centuries, a brand of polemics called "controversial theology" consisted of not much more than Catholics and Protestants telling the other why they were right and anyone not in their own church was wrong.

In the 19th century, for example, a German theologian named Adolf von Harnack saw Luther as a modern man striding into a new age of individualism where people owed their ultimate allegiance not to God or a nation but to their own conscience. For von Harnack, Luther was a trailblazer and a hero while Catholicism was stuck in miserable medieval centuries. As von Harnack wrote, Catholicism represented a "slavish dependence on tradition and the false doctrines of sacrament, of repentance, and of faith."

That opinion of Luther began to change as Protestant and Catholic scholars placed Luther back in his own Middle Ages. He may have been a prophet, but he was never anything more than human. Catholics, meanwhile, opened their minds to realize that, indeed, the medieval church did need a tune-up and had in some places and times veered far away from the life and teachings of a poor, Jewish carpenter from Nazareth named Jesus.

A great step that started with Unitatis Redintegratio in 1964 culminated on Oct. 31, 1999, when representatives of the Roman Catholic Church and the Lutheran World Federation signed a "Joint Declaration on the Doctrine of Justification" that recognized there is not much difference between Catholics and Lutherans on a key point: Christians are saved by faith through  grace — and grace is ultimately God's gift. Salvation can't be bought or earned, though we must all act in a manner where our good works match our words.

Not long after, however, some forces at the Vatican stressed that non-Catholic Christians are not quite as Christian as Roman Catholics. In place of meeting halfway, for these Catholics ecumenism really means "you-come-in-ism." More work needs to be done, to be sure, but what can we learn about the changes of the last 50 years?

First, if Catholics and Lutherans can see more of what they share than what they don't, can't other groups opposed to each other do the same: Christians vs. Jews vs. Muslims? Democrats vs. Republicans? 99 percenters vs. 1 percenters? Second, the way this change happened was through personal communication and openness: open hearts and minds, bridges instead of walls, dialogue instead of diatribe. We can meet face to face in place of slogan to slogan.

Finally, let's take a concept offered by a Lutheran theologian named Oscar Cullmann. He suggested Catholics and Lutherans approach their relationship with this phrase in mind: reconciled diversity. Pope Francis used the phrase himself on the feast of Pentecost earlier this year, noting that unity must always respect diversity while diversity must always respect unity. You cannot have one half of that sentence without the other. It's about balance, fairness, and justice. It's not saying, "I'm right and you're wrong." It's saying instead, "What do we share?" and then, "How can we resolve our differences that remain?"

Perhaps the greatest lesson of a monk named Luther — who, intentionally or not, brought division — is this: Over time and with care and respect, we might bridge the widest gaps through dialogue and mutual respect. We must come out of our bunkers first.

Christopher M. Bellitto is a professor of history at Kean University in Union, N.J.,  and author of Ageless Wisdom: Lifetime Lessons From the Bible (Paulist Press). cbellitt@kean.edu