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.