The Catholic Church is telling investors to think about avoiding the arms business. The advice comes in a document called “Mensuram Bonam,” or “Good Measure,” issued by the Pontifical Academy of Social Sciences, with an introduction by Peter Kodwo Cardinal Turkson. The document says it aims to offer guidance to those who work in finance for “more fully integrating the Church’s social and moral teaching into the management of their financial assets.”

The document sets up a clash between the world’s two most powerful Catholics, Pope Francis and President Biden. Mr. Biden has signed what, in nominal dollars, is the largest military budget in American history, and he has sent about $19.7 billion in military aid to boost Ukraine’s defense against the Russian invasion.

A table in the church document lists “armaments” and “nuclear weapons” among “categories of concern or prohibition,” along with abortion, contraceptives, and pornography.

An appendix explains the presence of armaments on the list by saying, “Military conflicts always cost human lives. The uncontrolled proliferation of arms facilitate many outbreaks of violence and erode secure peace. Thus, industries which thrive on the production of these instruments of war and destruction engage in a reprehensible business.”

Next to nuclear weapons, the appendix explains, “The Church’s teaching, as reiterated by Pope Francis, is that ‘the use of nuclear weapons, as well as their mere possession, is immoral.’”

As investment advice, staying away from arms manufacturers would have been costly, at least recently. The Wall Street Journal reported November 24 that even amid a general decline in the stock market, “Lockheed shares are up 36% since the start of the year. General Dynamics is up 22%, and Raytheon has climbed 12%.”

As Catholic churches in America struggle with declining church membership and declining attendance, it’s not clear that aligning the Catholic religion against the arms industry will be particularly popular. The American Revolution to free the country from colonial subjugation, the Civil War to end slavery and preserve the Union, and World War II to defeat the Nazis all relied on the production of arms. 

In some cases, an extreme stance against war may drive adherents away. The president of Brown University, economist Christina Paxson, told an Aspen Ideas Festival audience in 2018, “I grew up in a Quaker family, and going to Quaker meetings, and everybody was pacifist, it was during the Vietnam War … I decided at some point that I was not a pacifist.” Ms. Paxson eventually converted to Judaism.

Investing that takes into account environmental and social goals in addition to financial returns has been increasingly trendy in recent years, though the movement has also generated a backlash.

The Catholic Church, for its part, has oscillated over time between the claim that nonviolence is at the heart of the church and the competing support for “just war” theory arguing that in some cases war is morally defensible.

Pope Francis has tilted toward the nonviolent side. In a November 22 meeting with a Reagan administration Pentagon official, Ronald Lauder, and with other representatives of the World Jewish Congress, Francis said, “Together we recognize that war, every war, is always and everywhere a defeat for all humanity. I think of the conflict in Ukraine, a great and sacrilegious war that is threatening Jews and Christians alike, depriving them of their loved ones, their homes, their property, and their very lives.”

The Ukrainians fighting bravely to defend their country may be disappointed to learn that the pope considers their actions “sacrilegious.”

If investors take the Vatican’s guidance and avoid owning shares in the companies making the weapons Ukraine is using to defend itself, the Ukrainians may have a harder time holding off the Russians.

For investors whose decisions are driven by returns rather than pontifical pronouncements, however, any sales of weapons manufacturers by Catholic institutions, managers, or shareholders will be a buying opportunity. 

If invaders ever approach the Vatican, the pope would need a military defense. In that scenario, even the pontiff could find himself grateful that someone has stayed fully invested in the weapons business. As heroic as they might be, the Swiss Guards who protect the pontiff are armed with pikes and swords similar to the very weapons they carried when the Guards was founded in 1506.