For Mr Francois Fillon, his leading status in the French presidential election is becoming more than ever a matter of faith.

The 62-year-old former prime minister has led in polls since securing his right-wing party's nomination last November. He has campaigned on axing half a million civil service jobs, reining in France's welfare state and ending its infamous 35-hour working week.

Unusually for a French politician, however, he has also highlighted his religion - a move that has drawn the support of Roman Catholics.

But the man who once declared that "one can't lead France unless one is irreproachable" now needs them more than ever to keep their faith in him, after accusations this week that his wife was paid some €500,000 (S$762,000) in public funds for a no-show job.

Mr Fillon's recent profession of faith on national television may seem innocuous, but it marked a huge departure for a country that prides itself on a strict separation of Church and State.

It also feeds into a heated debate on France's identity at a time when questions are being raised about the place of Islam in the country.

Although France has a long Catholic tradition, bishops have over the past decades been struggling to get the faithful back to the pews. While 81 per cent of the French said that they were Catholics in 1978, only 64 per cent professed to share the faith in 2010.

The Church's influence is also curbed by laws guaranteeing that France remains a secular state.

But a group of vocal Catholic activists emerged in 2012 as a potent political movement, to oppose the Socialist government's plan to legalise same-sex marriage.

Hundreds of thousands took to the streets to join the protests, surprising many with their reach.

Crucially for Mr Fillon, a group that arose from those protests - called Sens Commun, or Common Sense - picked him as their man.

His crushing win in last November's right-wing primary led some analysts to suggest that the religious movement's backing had a major role in his success.

"Help! Jesus is back!" cried the left-leaning Liberation newspaper while the weekly L'Express noted the "awakening of the Catholics".

A staunch Catholic with five children, Mr Fillon has ruled out revoking the marriage law but pledged to amend it to prevent gay couples from adopting children.

He has also said he personally disapproves of abortion, even if he has no plans to repeal a 1975 law that legalises the termination of pregnancies.

For Mr Christophe Billan, the head of Sens Commun, Mr Fillon's remarks on the family chimed with his group's Christian agenda.

Standing outside the Notre Dame cathedral here before a recent Sunday mass, one voter, Ms Claire Lemoine, agreed.

"Mr Fillon is right to say he is Christian, as it lends us some backing and support," she told The Straits Times.

"For too long, the Catholics have been mocked, denigrated and considered backward."

Mr Fillon's support base is strongest in rural parts of western France.

Mr Paul Etienne, from the village of Carquefou, told The Straits Times that Mr Fillon can count on his vote come the election in April.

"I am voting for the one who's calmest, who has his feet on the ground and who has experience," said the 86-year-old retiree.

A recent poll by the French Institute of Public Opinion, commissioned by the Christian Family weekly magazine, found that almost half of practising Catholics - defined as those who attend Mass at least once a month - said they would vote for Mr Fillon in the first round of the elections.

His closest opponent, far-right leader Marine Le Pen, would get only a quarter of this demographic's vote.

To be sure, staunch Catholics are still a minority. A 2010 survey of French voters found that some 57 per cent do not attend Mass, even if they still get married in church and baptise all their children.

But after several terrorist attacks in France - in particular, the killing of a priest last July by supporters of the Islamic State in Iraq and Syria (ISIS) terror group - the issues of religion and France's identity have become part of the national debate.

If churchgoers previously had silent misgivings about government moves like strengthened abortion rights, "the terrorist attacks, the fact that the French are viewed as targets... brought about a reaction - a resurgence of the Catholic identity", historian Camille Pascal told Le Point weekly.

Mr Fillon has sought to strike a chord with voters on the issue of Islam in France.

In rallies, he has demanded "strict administrative control over Islam", which he said is meant to protect French Muslims from the ideology of "Islamic totalitarianism".

That demand aligns with the anti-immigrant, Islamophobic rhetoric adopted by Ms Le Pen, who has also accused Mr Fillon of playing the Christian card to win votes.

The Liberation daily viewed Ms Le Pen's attacks on Mr Fillon as stemming from her concern that he will erode her base of conservative voters.

"In 2012, 90 per cent of the Muslim vote went to the left. In 2017, the question is whether the Catholic vote will go to the right or to the far right," the newspaper said.

For Mr Fillon, an official investigation into the allegations that his wife collected a public pay cheque for several years without doing any work is a blow to a campaign built on an image of honesty, Christian values and government austerity.

He has denied any wrongdoing and said the probe would allow him to put an end to unfounded accusations.

Vowing to fight on with his bid for the presidency, Mr Fillon told broadcaster TF1 late on Thursday that the "only thing that will prevent me from being a candidate is... if I were indicted".

With polls showing Ms Le Pen eroding his lead, much will depend on whether his supporters' faith in him holds, as he prepares for his first major rally in Paris tomorrow - widely seen as the official launch of his presidential campaign.