A European Union flag, with a hole cut in the middle, flies at half-mast in the United Kingdom. (Photo by Christopher Furlong/Getty Images)

In their new book, “Religion and the Struggle for European Union,” the political scientists Brent Nelsen and James Guth explore an unlikely source of support for — and opposition to — the European Union: religion. Public opinion surveys from as far back as the 1970s show that Catholics tend to favor European integration; Protestants tend to resist it. As Europe becomes more secular, this trend has weakened, but it has not disappeared.

In a recent interview, Nelsen and Guth explained why Catholics are more sympathetic toward the E.U., why this may not last, and what this might mean for the future of the European project.

This interview has been edited for length and clarity.

NL& SW-L: Most people, when they think about factors that influence support for the European Union, don’t think about religion. What prompted you to do so?

Brent Nelsen: You’re right that, most of the time, people are skeptical, especially in Europe, about whether or not religion really is an independent factor influencing support for the E.U. But in 2001, we started looking at Eurobarometer data, and it’s very clear that Catholics, controlling for all other factors, favor the E.U. more than do Protestants. These attitudes were forged in the Reformation, with the development of two different approaches to governance in Europe. Catholics see Europe as a single cultural whole that ought to be governed in some coordinated way. Protestants, on the other hand, have seen the nation state as a bulwark against Catholic hegemony, and they have been very reluctant to give it up, even as religion has become less important.

NL& SW-L: What sort of gap are we talking about between Catholic and Protestant support for the E.U.?

James Guth: This varies by country. When you ask Protestants whether or not they identify with the E.U. flag — a marker of support for the E.U. — and whether they want that flag flown next to their national flag outside public buildings, about half of all Protestants say that they don’t identify with it or want it flown. Catholics are much more likely to say the opposite.

When you start taking other factors into account, that gap may decline a little. After the financial crisis and the refugee crisis, for example, support for the E.U. went down among Catholics rather significantly, whereas among Protestants in more prosperous areas, those attitudes didn’t decline very much. Still, Catholics remain more supportive of the E.U. than Protestants. In the past, among Catholics, the more religious you were, the stronger you supported the E.U. And that’s been true until very recently when some traditionalist Catholics have begun to rebel against the E.U. because of its liberal social policies.

NL& SW-L: What is it about Catholicism that promotes support for the E.U., and what is it about Protestantism that promotes the opposite?

BN: Catholicism has always been a universal religion. It was the successor to the Roman Empire, and in Catholic theology and ideology, there’s always been an emphasis on the unity of Christendom. Even today, even though the pope doesn’t claim secular authority, there’s still supranational governance within the Roman Catholic Church. So Catholics have always been very comfortable, even if subconsciously, with the notion of supranational governance.

After the Reformation, Protestants, on the other hand, attempted to carve out areas of religious liberty and caught on to the notion of the nation state. They didn’t invent the concept — it was invented by both sides as they came out of the religious wars of the 17th century — but the Protestants saw the nation state as very important for guaranteeing their liberty. For people in the Nordic states and the United Kingdom, the continent was the source of instability and of hegemony, and that’s part of why they developed a strong commitment to the nation and to national sovereignty — this was really the main vehicle for defense against, first, expanding Catholic control in the 16th and 17th centuries, and then, later on, Napoleon and Hitler.

NL& SW-L -Did religion play a part in the Brexit vote?

JG: Yes. If you look at the 2014 European Parliamentary Election Study, in the run-up to the Brexit vote, it’s clear that in the United Kingdom, Catholics were supportive of the E.U., as were minority religions — Muslims, Hindus, Buddhists — whereas Evangelical Protestants were the most critical of the E.U. And a lot of the surveys that were done just before and after the Brexit vote, even though they weren’t very good at identifying different religious groups, found pretty consistently that the more Protestant you were, the more critical you were of the E.U. That may have made the difference: If those Protestants had voted the way the average citizen of the United Kingdom had, Brexit wouldn’t have passed.

NL& SW-L: Was this religious split evident after World War II, when the idea of the E.U. was first being debated?

BN: The division has been clear from the beginning. Just look at the religious backgrounds of the E.U.’s founders. [Robert] Schuman, [Konrad] Adenauer, and [Alcide] de Gasperi were Catholic, and very devout Catholics at that. ([Jean] Monnet wasn’t so much — he only became a Catholic on his deathbed.) The Protestants, even early on, were very skeptical. The British did not contribute to the creation of the European Coal and Steel Community. At first they were cut out, then they didn’t want to be part of it, and then they observed for a while. They even created their own European free trade area. The Nordics, for various reasons, didn’t like what was going on on the continent. In Germany and the Netherlands, which have mixed confessional cultures, it’s the Protestants that are, if not overtly resisting the process, certainly trying to shape it in a less federal direction.

NL& SW-L: Do you observe the same trend when it comes to support for other supranational bodies, like the U.N. or the WTO?

JG: It’s a little harder to say. Often, there aren’t good measures of attitudes toward various supranational organizations that also measure religion. But in the evidence that does exist, you find that Catholics tend to be much more comfortable with the U.N. or the WTO, all other things considered. Some of these attitudes even cross the ocean. American Catholics are much friendlier toward the European Union than Evangelical Protestants are.

NL& SW-L: How do you know that the relationship between religion and support for the E.U. is a causal one?

JG: We tested all the alternative theories along with it. And most important, we have a theory that explains why religion has this effect: it has to do with Catholic comfort with supranational government, and it connects with Catholic social theology as it developed in Europe before and after the war. Of course, this doesn’t mean that all Catholics are supporters of the E.U. or that all Protestants are opponents. In fact, in the last decade or so, devout people of all traditions now tend to be more supportive of the E.U. In part, this reflects a change in attitude in state churches in a lot of Protestant countries that have become somewhat more supportive of the E.U. For example, in the United Kingdom, the Anglican Church has gradually moved toward a more affirmative position toward the E.U. (they took a cautious line in the run-up to the referendum, and almost all the bishops said that they were personally in favor of staying in the E.U.). It’s possible that devout Protestants are picking up on those cues, and now loyal Catholics and loyal Protestants seem to have moved into the same camp.

NL& SW-L: What other factors influence people’s support for the E. U.? How does religion stack up against them?

BN: Probably the main theory concerns economics. If you are going to benefit from the E.U. economically, the theory goes, you are going to support it. If not, you are going to resist it. People also think that support for the E.U. depends on educational levels, income levels, gender and how much you know about the E.U. We tested religion against all of those other factors, and we found that yes, there is some explanatory power in all these theories. But we also find that, controlling for everything else, religion remains a very strong contributor. In survey after survey, religion remains a factor in explaining attitudes toward the E.U.

JG: For a great many years, Catholic support provided an important bedrock for the E.U., regardless of fluctuations in the economy or in the political fortunes of the Christian Democrats or Social Democrats. But now, as Europe grows increasingly secular, that bedrock is eroding. This is particularly worrisome for the E.U. given its economic troubles and the fact that people are losing trust in their governments, which often indicates a lack of trust in the E.U. as well. And as those factors become more important, especially among young people, it becomes much more difficult for leaders to advance the cause of European integration.

NL& SW-L: Is Catholic support for the E.U. a result of explicit church guidance? Or is it simply an implicit cultural value?

JG: It’s both. The Catholic Church has explicitly supported European integration since World War II. Every pope since the end of World War II has been very supportive of the E.U. In 2014, Pope Francis gave a talk at the European Parliament about the need for the E.U. to rediscover its vision. Catholics are getting cues from the top, even if they’re subtle ones.

It’s the same story with Protestants. In the United Kingdom, you have Evangelical pastors who, on the Sunday before the Brexit referendum, were talking about how leaving the E.U. was the better Christian choice. I was at a conference in Oxford a couple of years ago, and on Sunday, I attended an Evangelical Anglican congregation. The greeter who met us at the door asked me what I was there for, and I explained that I was giving a paper on religion and European identity. He said, “Well, I think you’ve come to the wrong place. We don’t have any Europeans in this congregation.” People are getting cues like this all the time, from the clergy, from others in the congregation. It’s a pervasive cultural force, even if it’s becoming weaker.

BN: There’s also a broader force at work, a “confessional culture.” [The political scientists] Ron Inglehart and Pippa Norris talk about how religions influence culture — even as countries become more secular, the culture that religion shaped continues to influence people’s lives for a couple of generations. It’s background radiation.

NL& SW-L: How do people of other religions tend to feel about the E.U.?

JG: It’s hard to tell at the moment. Orthodoxy has a complex relationship with Europe, in part because it was Catholic Europe that became the E.U., and since the Great Schism, Orthodoxy has been a competitor. And some of the Orthodox churches have a connection with Moscow, which has used some Orthodox communions in Europe to try and undermine the E.U. in various ways. On the other hand, Orthodox folks often live in countries that really want to be in the E.U., often for economic reasons.

Muslims tend to be modestly more supportive of the E.U. than average. It may well be because they see the E.U. and its institutions as much more friendly toward ethnic and religious minorities than national governments. The E.U. has made a big effort to protect religious minorities. In addition, if you’re a Muslim in France or Germany or the United Kingdom, you may not feel the same kind of national loyalty or attachment as someone whose family has been there for generations and generations. So it may be a little easier to identify with a larger entity or institution. In general, religious minorities tend to be more supportive of the E.U. That was certainly the case in the Brexit referendum. If you put all the religious minorities together, they voted, as best as we can estimate, about 70 percent to remain.

BN: Minorities of any kind tend to be more supportive of the E.U. Even Protestant minorities in Eastern European countries tend to be friendlier toward the E.U. than Protestants in Protestant majority countries.

NL& SW-L: As Europe becomes secular and the E.U. loses some of its traditional Catholic support, is there any good news for European integrationists? Can a belief in international law and human rights provide an alternative source of support?

BN: In the postwar period, the founders wanted to create a European identity to provide a bedrock for a European polity. That didn’t work out so well. Then leaders thought they could ground European identity in the idea of human rights and Enlightenment values. But that seems like a weak grounding, especially as the E.U. has expanded eastward, where you find someone like [Hungarian Prime Minister] Viktor Orban, who rejects liberalism. So the bad news for the E.U. is that if it doesn’t succeed economically, it probably won’t succeed, because there’s not much that links all Europeans together emotionally.

JG: The one thing that might work in the E.U.’s favor is generational change. You saw in the Brexit vote that young people were much more inclined to stay in the E.U., and there is a clear tendency across Europe for younger people to be favorable toward the E.U. This may be because they have traveled more broadly and interacted with other Europeans through programs like Erasmus. But these sorts of transnational connections don’t make up for the lack of an ideology that captures the imagination of national publics. Especially if there’s no successful effort to revive European economies or deal with the migrant crisis, I just don’t see where that shared ideology will come from.