Pope Francis is continuing along his remarkably liberal path, most recently by praising Martin Luther at a ceremony in Sweden beginning a yearlong 500th anniversary commemoration of the Reformation. Yet despite the pope’s openness, and the corresponding good faith of the Lutherans, the two sides were unable to effect a reconciliation. The episode raises two questions: Why try? And how is it that, in this post-theological age, not even Christian believers can get past their own wars of religion?

To start with the obvious, it’s moving that the pope would speak positively about Martin Luther, who is technically speaking still under a ban of excommunication issued by Pope Leo X in 1521. In 2011, Benedict XVI offered some mild approbation of “Luther’s burning question” -- about the nature of justification, or salvation, in Christ. But Francis went further, saying that Luther’s issue is “the decisive question of our lives,” and that his doctrine of justification “expresses the essence of human existence before God.”

In case you’re a little rusty on your soteriology, Luther’s transformational argument about salvation can be summed up in the slogan “faith alone.” God grants salvation as an act of free grace based on the believer’s faith in Christ, not the believer’s actions. In contrast, the Catholic Church believes in salvation through a combination of faith and acts.

Despite what would appear to be significant differences, the Catholic Church and a raft of Lutheran churches signed in 1999 a “Joint Declaration on the Doctrine of Justification” that enabled them to claim that they now share “a common understanding of our justification by God’s grace through faith in Christ.”

I’ll spare you the doctrinal heavy lifting. What’s significant about the document is that it shows churches can find common ground when they want to. After the joint declaration, it seemed conceivable that Catholics and Lutherans might actually be able to repair their schism and enter into communion with each other.

Why bother to try? Part of the answer is undoubtedly theological. Both Catholics and Lutherans are committed to the idea of the church as a single mystical body. They therefore consider the 500-year-old breach a “wound in the body of Christ,” as the two sides agreed to put it in a joint statement this week.

But the desire for union goes further. In an era when Islam seems to be rising, and Christianity finds itself on the defensive, especially in rich Western countries, resolving internal conflict has public relations value. It’s embarrassing for a faith that wants to teach love, peace and harmony to find itself still divided by disagreements that produced hundreds of years of war and violence. The continuing division between Catholics and Protestants is a reminder that disagreement over what appeared to be small differences can be pervasive and violent.

Given the value of reconciliation, why can’t Catholics and Lutherans enter communion with each other? At the level of theology, the problem isn’t the doctrine of justification, but the other classic divide between the two churches: belief about what happens during the Eucharist, when the bread and wine consecrated by the priest become in some sense the body and blood of Christ. The Catholic Church believes in transubstantiation, according to which the substance or essence of Christ’s body and blood are present in the bread and wine at the moment of communion. The Lutheran doctrine of “sacramental union” is a bit different and more complicated. The body and blood are really present, but not in the same way as the bread and the wine.

This theological divide could be bridged. Luther did not take the view, adopted by many other Protestants, that the holy supper of communion is symbolic only, or that Christ’s body and blood are not real presences at the altar. Indeed, it’s fair to say that the two churches are closer together on communion than they were on the doctrine of justification, which they “resolved” by their joint declaration.

So why can’t the Catholic and Lutheran churches get to yes? The answer lies in 500 years of institutional development, not in theology. Churches, like other institutions, develop strong identities and affiliations that go far beyond belief. To be a Lutheran is, in no small part, not to be a Catholic. To be a Catholic is, importantly, to belong to the church that rejected Luther’s Reformation.

As a result, priests and lay people alike would find it all but impossible for the two churches to say they were in communion. The divisions are at the level of fundamental identity. The culture of worship plays some role -- Catholic velvets and silks are different from plain Lutheran black and white. But even these divisions matter mostly because of how they relate to identity.

The upshot is that Catholics and Lutherans are in this sense like Sunnis and Shiites -- divided more by identity and history than by theology or belief. The good news is that they aren’t at war, and haven’t been for centuries. The bad news is that identity divisions created over centuries can take centuries to fade away.

By Noah Feldman

Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to US Supreme Court Justice David Souter. –Ed.

Bloomberg