Like many proud, East Coast WASP institutions, the Episcopal Church has lost considerable clout over the last few decades. Trinity Church, in Manhattan, is a rare redoubt of influence. Thanks to Queen Anne's 215-acre donation in 1705, Trinity is one of Manhattan's largest landholders. The church has sold some of the land and parlayed into a $2 billion portfolio, making it an exceptionally wealthy parish.

Two years ago, an internecine fight broke out over what to do with that money. Some members of the vestry, the parish's managing board, felt the church should be more activist, and many of the more passive members resigned or were forced out.

In a sign of how that fight shook out, Forbes's Clare O'Connor reports Monday that Trinity is using a small piece of its portfolio to pressure Walmart on gun sales. The church owns a token amount of stock in the mega-retailer—$2,000—but it wants shareholders to vote on a proposal that's effectively an anti-gun measure. In short, Trinity says, Walmart's board should decide whether the chain will sell certain rifles. Walmart chooses not to sell plenty of other products for moral reasons; why not guns with high-capacity magazines?

"There is a substantial question regarding whether these guns are well suited to hunting or shooting sports; it is beyond doubt that they are well suited to mass killing, and tragically more effective for the latter purpose, than are the handguns equipped to fire ten or fewer rounds that the company chooses not to sell except in Alaska," Trinity wrote. The church is maintaining a careful distance—it's not saying it wants to decide, but it's saying Walmart's board ought to do so. Walmart tried to bury the proposal; the Security and Exchange Commission ruled in the company's favor. Trinity sued and won in federal district court, and now Walmart has appealed.

What Trinity lacks in substantial holdings it partly makes up for in moral standing: Who wants to be seen fighting against a church, especially one that became a national icon for standing in the shadow of the Twin Towers on 9/11? But while Trinity's historical standing and massive endowment place it well out of the ordinary, its political stand is in line with many mainline Protestant churches. In fact, a coalition of mainline Protestant denominations, the Catholic Church, and Jewish and Muslim leaders are now among the staunchest institutional backers of stricter gun controls. That stands in contrast to evangelical Christians in America, who are far more divided on the gun question.

The Episcopal Church has shed some of its old-guard image and become an unlikely champion of progressive causes—approving gay unions, electing an openly gay bishop, and choosing a woman as presiding bishop (the top job) when the home church in England didn't even allow female bishops. In 2013, Presiding Bishop Katharine Jefferts Schori called on Episcopalians to demand congressional action on gun violence.

Gary Hall, the dean of the Washington National Cathedral, has also used the church's prominent position to advocate forcefully for stricter gun laws. Hall sees this sort of advocacy as a religious imperative. "Christians follow someone who died at the hands of human violence. They didn’t have handguns in his day, but the cross is essentially a form of human torture and execution," he told me in 2013. "For me the great reason that gun control is a theological issue is because it's tied to the church's compassion for victims of violence, and it's tied to the fact that part of our job is to be a community that heals the world."

The Catholic Church has also been outspoken about the need for stricter gun laws, construing it as a pro-life issue akin to abortion. The U.S. Conference of Catholic Bishops backs a ban of assault rifles, stronger background checks, and limitations on high-capacity magazines. In 2013, after the Newtown shootings, the Public Religion Research Institute found that 67 percent of Catholics favor stricter gun laws.

"The injustice of taking innocent life lies at the heart of the church’s pro-life position," Sister Mary Anne Walsh, a spokeswoman for the USCCB, wrote in The Washington Post in 2013. "There is no question about the innocence of pre-born children. And Americans are becoming more and more uneasy as we learn of people on death row eventually found innocent of the crimes for which they were convicted."

The Islamic Society of North America has favored stricter gun laws, citing the Qur'an. The Union of Reform Judaism has long argued for stronger laws, and reiterated that stand in the wake of the Newtown shootings, as did the United Synagogue of Conservative Judaism.

Among other mainline Protestant denominations, the United Methodist Church has taken a pro-gun-control stance. So has the Presbyterian Church USA, the larger and more liberal of two branches of the church in the U.S. The Evangelical Lutheran Church in America went on the record in favor of controls in 1993. The churches are more uniform in their views than their congregations, but 57 percent of white mainline Protestants favor stricter laws, PRRI found. A whopping 76 percent of black Protestants agreed.

Of course, these populations tend to be those that support liberal politics more broadly. Mainline Protestants tend to be better educated, which correlates with support for gun control. Black and Jewish voters also trend consistently progressive.

And the rub for gun-control advocates—and for mainline Protestants—is that Protestant denominations have seen a steady decline in numbers and influence in American society. Evangelical Protestants, however, are a growing portion of the U.S. religious landscape, and they're far more divided on gun control than their mainline and Catholic brethren.

PRRI, in that 2013 poll, found just 38 percent of white evangelical Protestants in favor of gun control, with 59 percent opposed. Yet about three-quarters of the leaders at a National Association of Evangelicals meeting around the same time said they favored stronger laws. This may be a case where, as among mainline denominations, pastors are out in front of their flocks on the gun question. Moreover, the diffuse nature of American evangelical Christianity, without the hierarchical structures that characterize the Episcopal or Presbyterian Churches. Writer Jonathan Merritt has wished publicly for Rick Warren, the evangelical superstar whose son committed suicide with a gun, to lead a movement among evangelicals for stricter laws, but no such push has crystallized so far.

Even without uniform support across the board, American faith communities remain the most steadfast institutional supporters of gun control. Of course, what gun-control advocates have discovered over the last two decades, especially at the national level, is that having a great deal of support doesn't always translate into material successes. In the Walmart case, we'll see how effective moral authority can be.