When French presidential contender François Fillon marked the Feast of the Assumption last summer, he attended Mass at Solesmes Abbey, a Benedictine monastery known for resisting the anticlerical purges of the French Revolution. The trip, coming just weeks after the slaying of a Catholic priest in a terror attack, didn’t go unnoticed.
“He doesn’t hide the fact that he’s Catholic,” said Christophe Billan, head of Sens Commun, a grass-roots movement comprising thousands of French Catholics.
In France, the strict separation between personal faith and public life, known as laïcité, is a pillar of national identity. However, a confluence of events—from the legalization of gay marriage to the more recent string of Islamist terror attacks—has many conservative voters looking to the country’s Christian heritage as a bulwark.
Mr. Fillon’s candidacy is seizing on that impulse. In publicly embracing his faith, the 62-year-old is tapping a wellspring of Catholic voters who have begun coalescing into a potentially decisive voting bloc.
His performance during the country’s first-ever conservative primaries provided the clearest sign yet of the revived Catholic vote. After lagging behind rivals for weeks, Mr. Fillon spent the homestretch of the race debating opponent Alain Juppé over which of them stood closer to the teachings of Pope Francis —a development Le Monde described as “unprecedented.”
More than two-thirds of the people who voted in the primaries described themselves as Catholic in exit polls, and they helped hand Mr. Fillon a commanding victory. Pollster OpinionWay said 83% of Catholics who regularly attend Mass voted for Mr. Fillon and 68% of nonpracticing Catholics also backed him. Between 55%-60% of the overall French electorate identifies as Catholic, according to Jerome Fourquet, director of polling firm IFOP.
“I’ve never been so consciously influenced by my being Catholic,” said Catherine Mordant, 46 years old, a stay-at-home mother of four children who voted for Mr. Fillon. “Now we have to act, because the problem is really crucial.”
The Catholic vote is shaping up to play an unusually prominent role in the general election in May, when polls predict Mr. Fillon will face-off against Marine Le Pen , leader of the far-right anti-immigrant and anti-euro National Front party.
Many conservative Catholics shifted to the National Front during recent regional elections, feeling more at home with its call for revived nationalism than with the pro-EU principles—free movement of people and goods—espoused by other parties.
A quarter of self-described practicing Catholics voted for the National Front in December 2015 regional elections, up from 16% in local races in March of that year, according to IFOP.
Mr. Fillon’s Catholicism reassures voters who want to show support for French traditions. “The National Front has made a lot of progress with this group,” said Mr. Fourquet. “They could come back to the center-right with Fillon.”
The rise of a Catholic vote in France is a measure of how deeply the continent has been shaken by a series of crises, from the arrival of migrant waves from the Middle East to the surge in political parties questioning the future of the European Union itself. Just over a decade ago, it was France that led a successful campaign to prevent any reference to Europe’s Christian heritage from being added to the European Union’s constitution.
Today the EU is grappling with nationalist movements that point to President Vladimir Putin of Russia as a model of leadership, preferring his authoritarianism to the uncertainty clouding the economic bloc. Mr. Fillon has cultivated ties with Mr. Putin, criticizing sanctions the EU imposed on Russia after its forced annexation of Crimea.
Mr. Fillon has been careful to couch his talk of faith in language respectful to secularism. His support for Church teachings are personal choices, he says, not policy prescriptions. He has said he is personally against abortion but believes pro-choice laws shouldn’t be changed, and that he wouldn’t repeal the gay-marriage law but would revise sections that legalized adoption by gay couples.
Still, the politician has gone further than many of his peers in demanding space for religious voices in the public square. “Whenever the nation faces fundamental questions—life, death, what makes us human beings—it’s important that the point of view of religions not be ignored,” Mr. Fillon wrote in a chapter dedicated to faith in his book “To Do.”
In September, he returned to the question of religion and Republican values with the publication of a follow-up, best-selling volume, “Vanquishing Islamic Totalitarianism.”
“Let’s stop kidding ourselves,” he wrote. “France doesn’t have a problem with religion [in general]. The problem is linked to Islam.”
French secularism grew out of the 18th-century Enlightenment and the 1789 Revolution. It was codified in a 1905 law on the separation of church and state that strictly limited the display of religious symbols in public places and forbade religious instruction in public elementary schools.
Designed to curb the influence of the Catholic Church, the law also helped lay the foundations for political conduct in the post-World War II era. French Catholics followed the cues of statesmen from Charles de Gaulle to François Mitterrand, who mainly kept their religious beliefs to themselves.
Any public references to faith were discreet. Mr. Mitterrand was praised for a 1981 campaign poster that set him against a bucolic background dominated by a church bell tower—a symbol of the central place of Christianity in the secular nation’s heritage. At the same time, the church’s cross had been airbrushed out.
The balance between public service and private faith has come under strain as the children and grandchildren of North African immigrants in the 1960s have come of age. These younger generations of one of Europe’s biggest Muslim minorities tend to practice stricter forms of Islam.
In response, successive French governments have become increasingly strict in their application of secularism. A debate over students wearing Islamic head scarves led to a 2004 ban on wearing religious symbols in general in public schools, including crosses and yarmulkes.
Catholics who once steered clear of politics out of respect for laïcité gradually found reason to speak up. One moment came in 2013 after newly elected president, Socialist François Hollande, signed legislation legalizing gay marriage. To the surprise of many, hundreds of thousands of Catholics took to the streets in what was known as a “manif pour tous,” a protest march for everyone.
“A cornerstone was being touched—defining the identity of the child, the couple—and we were barred from the debate,” said Mr. Billan of Sens Commun.
Seizing on the momentum of the protests, Mr. Billan and others founded the grass-roots movement, called “common sense,” with 9,000 members across the country. Though not officially Catholic, the group aimed to pressure lawmakers on a platform consistent with church teachings. Suddenly, French Catholics had a lobby.
The group found a kindred spirit in Mr. Fillon. He had grown up in Sarthe, a rural area nestled in France’s northwest, where he attended a Jesuit school. He recited morning prayers and mealtime benedictions.
“I grew up in a world where the Catholic faith structured whole sections of your social life,” Mr. Fillon wrote in “To Do.”
As prime minister between 2007 and 2012 to then-President Nicolas Sarkozy, Mr. Fillon’s social conservatism took a back seat to his role as a technocrat carrying out economic policy.
When he returned to the opposition as a lawmaker in 2012, however, Mr. Fillon clashed with Mr. Hollande’s Socialist government. He voted against the gay-marriage bill and criticized the government for not doing more to protect Christian minorities in Syria, Iraq and other parts of the Middle East, organizing a rally in June 2015 to support them.
“We are all Eastern Christians!” Mr. Fillon told the crowd.
A year later, Mr. Fillon met with Mr. Billan of Sens Commun, seeking the group’s support to better compete with Messrs. Sarkozy and Juppé, who had the support of the machinery of the conservative party, the Républicains.
Sens Commun had built the kind of grass roots organization Mr. Fillon lacked. It had phone banks, a social-media operation and local chapters across the country that would eventually be called upon to canvass for voters and drive them to the polls.
Weeks later France was hit by a pair of terrorist attacks. The first, a truck attack on a Bastille Day fireworks display in Nice that killed 86 people, struck at a symbol of national unity.
Less than two weeks later, the brutal slaying of Rev. Jacques Hamel, 85, while he celebrated Mass in a small town church in the country’s north stirred a rare outpouring of support for France’s Catholic roots. Thousands of people, including Mr. Fillon, packed into Notre Dame of Paris to celebrate a Mass in tribute to the priest.
Thibault Fraisse, a 28 year-old doctor from the town of Aurillac in central France, said he worried the priest’s slaying and other attacks were an outgrowth of Muslim communities isolated from the rest of French society. He said wider acknowledgment of France’s Christian past, and a vote for Mr. Fillon, could act as a counterweight.
“We have to recognize that France is first and foremost a country with Catholic roots,” said Mr. Fraisse, who describes himself as a nonpracticing Catholic.
In August, Mr. Fillon held a rally near his hometown, where he warned of a France “ashamed” of its history and reminded the crowd he had recently celebrated the Feast of the Assumption at the nearby Abbey of Solesmes.
“You just heard the bells ringing,” Mr. Fillon said, gesturing toward the Benedictine monastery. “A thousand years of history! How can you not feel the force, the power, the depth of this past that forged us, that giv