“It deeply contradicts secularism and our values,” Ms. Le Pen went on. “To justify a political choice with religious beliefs is shocking: How will we oppose those who, tomorrow, will want to enact policy in the name of their faith — like, for instance, Islam?”

The debate will not go away. Catholics, who took part in mass demonstrations against legalization of gay marriage three years ago, are emerging as a political force in this campaign. In the Republicans’ primary in November, candidates discussed which one of them was closer to Pope Francis’ views on social issues. Campaigning this month in the Socialist primary, the former prime minister Manuel Valls, challenged by a young Muslim woman on the issue of the Islamic veil — which he views as an enslavement of women — described France as “a country with Christian roots that hosts the oldest Jewish community in Europe.”

This fresh enthusiasm for Christianity has less to do with God, though, than with culture and identity. Polls usually show that close to 55 percent of French citizens describe themselves as Roman Catholics (the rest being divided among Muslims, Jews and Protestants), but only 5 percent to 8 percent go to church regularly. An Ipsos study recently commissioned by the Catholic media group Bayard has created a new category of believers: “committed Catholics,” people who don’t necessarily attend church but identify with the Catholic Church through philanthropy, family life or social involvement. This group is said to include 23 percent of the French population.

Though they represent a variety of opinions on matters from migrants to Pope Francis or political orientations, this group can be seen as a potential electoral bloc. “These cultural Catholics have been under the radar screen because polls did not identify them, and because secularized political and media elites did not see them,” Jean-Pierre Denis, the editor of the Catholic weekly La Vie, told me. The socially conservative Mr. Fillon, he said, “has been smart enough to spot them and tap into them.”