In his victory speech, Joe Biden pledged “to be a president who seeks not to divide but unify. … And work with all my heart … to win the confidence of all of you.” If the presidential campaign is any indication, Biden may have the most challenging time winning over one group of Trump voters: conservative Catholics.

This may be surprising, given Biden’s devout Catholicism and the role it plays in his political philosophy. Yet, even as Biden attended church during the campaign and talked about his faith, his conservative coreligionists charged him with not being a “real” Catholic. Two days before Election Day, protesters focused on abortion picketed outside of Biden’s Delaware church as he attended Mass. One woman’s sign read, “Biden No Devout Catholic Supports Abortion.” Some chanted, “Repent for your soul” and “Repent for Beau’s soul.” As Biden exited the church, two people yelled, “Joe, you’re a disgrace to the Catholic faith.”

As jarring as this may be, intra-Catholic squabbles in the political arena are nothing new. During the Obama administration, for example, Catholics sparred over the Affordable Care Act. Abortion tends to be the dividing line in many of these debates.

Yet, an example from foreign policy best illustrates the challenges ahead for Biden. During Ronald Reagan’s presidency in the 1980s, liberal Catholics opposed U.S. intervention in Central America, while conservative Catholics helped craft U.S. policies. The fight was particularly nasty because the policy battle was a proxy war for larger debates within the church, exactly what Biden will face in the years to come.

The divide among Catholics dates to Vatican II, the worldwide council of Catholic bishops held from 1962 to 1965, which prompted a reexamination of the church’s role in society and what it meant to be Catholic, while ushering in seismic changes to Catholic practices. To the average Catholic, the most significant — and noticeable — difference involved the way most celebrated Mass. Vernacular replaced Latin and the priest turned to face the congregants. Catholics worldwide split in their response.

In general, during the 1960s, U.S. Catholics who welcomed these liturgical changes also supported civil rights and questioned U.S. anti-communist policies, whereas those who opposed changes to the Mass opposed the civil rights movement and were fervent anticommunists. When some nuns and priests marched in support of civil rights, other Catholics condemned them as social activists and even communists. When some Catholics opposed U.S. involvement in Vietnam, others criticized them as rejecting the Catholic Church’s anti-communist stance, which dated to the 1800s.

Liberation theology was another major source of these tensions. Developed in 1960s Latin America, this social movement and school of thought borrowed from Marxist analysis, and understood sin in terms of the oppressive societal structures from which people needed salvation. It was not God’s will for people to be poor; man-made societal structures created inequalities.

Liberation theology inspired revolutionary movements, particularly in Nicaragua and El Salvador. To conservative Catholics, liberation theology threatened both the state and the church. In the United States, they also worried that liberation theology would infect the church, via returning U.S. missionaries.

These divisions burst into the spotlight on Dec. 2, 1980, when Salvadoran national guardsmen raped and murdered U.S. missionaries Maryknoll Sisters Maura Clarke and Ita Ford, Ursuline Sister Dorothy Kazel and lay missioner Jean Donovan. The women had been aiding refugees fleeing El Salvador’s civil war. Their deaths sparked outrage and fueled a growing protest movement against U.S. policy, as more Americans learned that the United States was supporting and arming the Salvadoran government. As one protest slogan declared, “U.S. Guns Kill U.S. Nuns.”

But not all Americans — or Catholics — agreed. After the killings, Jeane Kirkpatrick, soon to be Reagan’s ambassador to the United Nations, falsely charged that the women “were not just nuns. … They were political activists” supporting guerrilla fighters. Months later, the new secretary of state, Catholic Alexander Haig, testified before Congress that the women “may have tried to run a roadblock” and died in a shootout. The claim was so outlandish that even the FBI privately distanced itself from it.

It would be easy to dismiss Haig as a loose cannon, given his infamous response after the attempted assassination of Reagan and his short time in the administration, but he voiced what many conservative Catholics publicly argued: The women were asking for it by not behaving like proper nuns or proper Catholics. To conservative U.S. and Central American Catholics, these women, and others like them, were agents of class conflict. They were inciting revolution among the poor.

Liberal Catholics, by contrast, viewed the women as models of faith, worthy of emulation as human rights advocates. They lived out the Gospel spirit by encouraging the poor to value themselves and by working to combat structural inequality.

These disagreements not only showed how faith and U.S. foreign policy intersected, but also how deep-seated Catholics’ differences regarding the church, its place in society and its very teachings were.

Although these dividing lines had not changed since the 1960s, in Reagan, an ardent conservative and anti-communist — like the new pope, John Paul II, who ascended in 1978 — conservative Catholics found a political ally. Although conservatives were the minority among U.S. Catholics, administration officials and high-level advocates such as Paul Weyrich and Phyllis Schlafly gave them outsize political influence.

They fashioned policies that reflected their religious and political views. Reagan considered the Salvadoran guerrillas and the Sandinistas threats to U.S. national security; conservative U.S. and Central American Catholics agreed, and also regarded them as threats to the church. Violence against fellow Catholics was justified — and necessary — to preserve the state and the church. They were viewed as heretics and traitors. Reagan bolstered conservatives in the battle over the church’s direction by providing financial support and military aid to the Salvadoran government and the counterrevolutionaries (contras) fighting the Nicaraguan government. His stance paid off politically. He earned more Catholic votes in 1984.

In 2020, abortion, not Central America, served as the flash point for Catholic voters. Those who prioritized abortion as the first — and only — issue supported President Trump, whereas Catholics who contended that respect for life should mean something broader, such as opposition to the death penalty and to separation of children from their parents at the border, sided with Biden.

But the roots of the division remained the same: To conservative Catholics, it is not just Biden’s stance on abortion that angers them, it is the kind of Catholicism he represents. It is no accident that Biden references Pope Francis while Trump supporters placed the president’s photo next to John Paul II in campaign materials. It is as though there are two churches. The groups have fundamentally divergent views of what their faith entails, yet both strongly value their faith. Their stances underscore how political and religious identities are not easily untangled, whether one is an ordinary voter or the president-elect.

If the debates of the 1980s are any indication, conservative Catholics probably will present Biden’s biggest challenge politically and personally, because they will oppose his policies by attacking his faith, which is so central to him. Biden, in turn, will respond based on his policy views and his faith.

This infighting will be more heated than in the 1980s, because despite his many conservative Catholic advisers and allies, Reagan, as a non-Catholic, was an outsider. While Trump addressed religion transactionally, trading policies near and dear to religious voters in exchange for support, for Biden, these rifts will be far more personal. The coarsening of political discourse, often amplified via social media, will only exacerbate the attacks.

In trying to save the “soul of the nation,” and implement policies derived from his own deeply held faith, the new president will be wading into the fundamental dispute about what Catholicism should be. This may make it difficult for Biden to achieve his goals, both of healing the nation and implementing his agenda.