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Pope Francis has warned the Catholic Church would become "a compassionate NGO" without spiritual renewal.
In a Sistine Chapel Mass with cardinals on his first day as Church leader, the pontiff said: "If we do not confess to Christ, what would we be?
"We would end up a compassionate NGO. What would happen would be like when children make sand castles and then it all falls down."
Francis is the first Latin American - and the first Jesuit - Pope.
The term used by the Pope is the abbreviation often used for a non-governmental organisation, such as a charity or activist group.
The BBC's David Willey, in Rome, says the 76-year-old has already been swift to stamp his style on the papacy.
Pope Francis is regarded as a doctrinal conservative, but he is also seen as a potential force for reform of the Vatican bureaucracy, analysts say.
Shunned special car
On Wednesday night, Pope Francis endeared himself to the crowds in St Peter's Square - and underlined his reputation for humility - when he asked them to bless him before blessing them in return from the balcony of the basilica.
The Vatican's account of his first hours in the top job on Thursday also emphasised Pope Francis's humility, describing how he shunned a special car and security detail provided to take him to the Vatican, travelling instead on a bus with the other cardinals.
Following his first outing as pope to the Rome basilica on Thursday, Francis went back to the clergy house in a city centre side street where he had been staying ahead of the conclave that elected him on Wednesday.
"He packed his bags and then he went to pay the bill for his room so as to set a good example," said Vatican spokesman Father Federico Lombardi.
He also broke tradition by remaining standing to receive cardinals' acts of homage after his election, instead of sitting in the papal throne, Father Lombardi said.
On Friday, Pope Francis will meet all the cardinals, including those aged over 80 who did not take part in the conclave.
On Saturday he will meet the world's media at a special papal audience, an opportunity perhaps to set out some of his global vision, says the BBC's James Robbins in Rome.
A visit to his predecessor Benedict XVI at his retreat at Castel Gandolfo outside Rome is also planned, but will not take place in the next couple of days, Father Lombardi said.
The visit to Benedict is important, correspondents say, as the existence of a living retired pope has prompted fears of a possible rival power.
Francis will be installed officially in an inauguration Mass on Tuesday 19 March, the Vatican added.
Force of reform?
The election of Cardinal Jorge Mario Bergoglio surprised many observers when it was revealed on Wednesday.
Although he reportedly came second to Pope Benedict XVI during the 2005 conclave, few had predicted the election of the first pope from outside Europe in 1,300 years.
Despite his reputation as a doctrinal conservative, Pope Francis is also seen as a potential force for reform of the Vatican bureaucracy - and analysts say that may have won him the support of reforming cardinals.
The new pontiff will certainly come under strong pressure to reform the Curia, the governing body of the Church.
He will also face an array of challenges which include the role of women, interfaith tensions and dwindling congregations in some parts of the world.
The 76-year-old from Buenos Aires is the first Pope to take the name of Francis - reminiscent of Francis of Assisi, the 13th Century Italian reformer and patron saint of animals, who lived in poverty.