Pope Benedict XVI waded into French politics yesterday, throwing his weight behind a controversial drive by President Sarkozy to put religious faith back into the life of the strictly secular state.

An hour after President Sarkozy and Carla Bruni, his wife, had greeted him at Orly airport, the Pope told the Cabinet and opposition leaders in the Elysée Palace that he shared the President's view that politics and religion must be open to one another.

The Pontiff's remarks opened a difficult four-day mission to rekindle enthusiasm for the Roman Catholic faith in one of the world's least godfearing nations. Fewer than five per cent of the historically Roman Catholic French attend services regularly. Fifteen per cent of the French call themselves atheists - a figure that is double the European average. Up to half a million people are, however, expected to turn out for open air masses on the Left Bank of Paris on Saturday, and at the shrine of the Virgin Mary at Lourdes on Sunday.

"At this moment in history when cultures continue to cross paths more frequently, I am firmly convinced that a new reflection on the true meaning and importance of laicité (secularism) is now necessary," said the German-born Pope, who is a far more fluent French speaker than John Paul II, his predecessor.

Separation of church and state was necessary, he added, but societies must also be "more aware of the irreplaceable role of religion for the formation of consciences and the contribution which it can bring to ...the creation of a basic ethical consensus."

That thought might be unremarkable elsewhere, but in France it is seen to infringe the law of laicité -- strict secularism -- first established with the bloody purge of the clergy in the 1790s and confirmed in 1905 when the state took over church property.

Mr Sarkozy has stirred the wrath of the left and part of the establishment with a drive for what he calls "positive secularism". He used the Pope's first visit to France to hammer the message today. Some opposition leaders even accused him of breaching the law by inviting a religious leader into the Elysée palace. They also objected to Mr Sarkozy's airport trip to greet him, the first time that the President has conferred that honour on a visiting leader since taking office in May 2007.

The President, who is twice divorced and a self-described lapsed Catholic, said in his palace welcome to the Pope that it would be "folly to deprive ourselves of religion."

Spirituality was "not a danger for democracy, not a danger for secularism," he said. "We do not put anyone above anyone else, but we accept our Christian roots. That does not stop us from doing everything to ensure our Muslim compatriots can live their faith equally with all others."

The argument is explosive for supporters of laicité partly because it tampers with the hard-fought consensus that keeps religious practice out of public life. This includes the 2005 ban on religious dress in state schools, which was mainly meant to stop girls from France's six-million Muslim community covering their heads.

Mr Sarkozy persuaded the Pope, who is an expert in modern French literature, to come to Paris before visiting Lourdes in order to talk to the government and artists, intellectuals and scientists.

The trip is difficult because the Vatican sees France as one of the least obedient nations among its senior flock, a group that includes the Americas, Poland, Ireland and the southern European nations. Just over half the French still call themselves Catholic but as divorce, contraception and abortion have become routine, church attendance has shrivelled in recent decades. A majority of French children are born outside marriage.

The Vatican faults its French church leaders for failing to market the faith more vigorously in the face of the state-enforced laicité. Monsignor André Vingt-Trois, the Archbishop of Paris, denied that the Pope had come to deliver a pep talk and he insisted that "the Church is not a field of ruins". "I do not see him coming to tell us to pull up our socks," he said. "The Church in France is not gravely ill; it is even seriously alive."

Cardinal Vingt-Trois conceded that "For the French, the Pope is still John Paul II. ...It has to do with the two men's personalities. Benedict is not a man for the crowds. He is a very private person."

While congregations have dwindled, Church members point to a new activism among younger believers who are focusing on faith rather than tradition. These new activists, however, are uneasy about the conservative, German-born Pontiff who was known as the PanzerKardinal when the he was the Vatican's doctrinal enforcer.

The Pope is visiting to Lourdes as part of the 150th anniversary of apparitions of the Virgin to Bernadette Soubirous, a 14-year-old girl. The Catholic Church says the apparitions were genuine but the Pope has ordered officials to draw up new guidelines for bishops around the world concerning the recognition of other reported apparitions. He wants to avoid "excesses and abuses" and wants "scientific, psychological and theological criteria" to be applied to their certification.