In his first public Mass, Pope Francis urged the Catholic Church to stick to its Gospel roots and shun modern temptations, warning that it would become just another charitable group if it forgot its true mission.
In a heartfelt, simple homily, the Argentinian pope laid out a clear moral path for the 1.2 billion-member church.
Addressing cardinals in the frescoed Sistine Chapel the day after his election there, Jorge Bergoglio said the Church should be more focused on the Gospels of Jesus Christ.
"We can walk all we want, we can build many things, but if we don't proclaim Jesus Christ, something is wrong. We would become a compassionate NGO and not a Church which is the bride of Christ," he said, speaking in Italian without notes.
The first non-European pope in 1,300 years, Bergoglio's initial steps suggested he would bring a new style to the papacy, favouring humility and simplicity over pomp, grandeur and ambition among its top officials.
Whereas his predecessor, Pope Benedict, delivered his first homily in Latin, laying out his broad vision for the Church, Francis adopted the tone of parish priest, focusing on faith.
"When we walk without the cross, when we build without the cross and when we proclaim Christ without the cross, we are not disciples of the Lord. We are worldly," he told the massed ranks of cardinals clad in golden vestments.
"We may be bishops, priests, cardinals, popes, all of this, but we are not disciples of the Lord," he added.
Earlier, Pope Francis had quietly slipped out of the Vatican to pray for guidance at one of Rome's great basilicas before returning briefly to a Rome hostel, where he had left his bags before entering the secret conclave on Tuesday.
Francis, who has a reputation for frugality and an understated lifestyle, insisted on paying the bill. "He was concerned about giving a good example of what priests and bishops should do," a Vatican spokesman said.
Father Pawel Rytel-Andrianik, who lives in the same residence in the winding backstreets of central Rome, said: "I don't think he needs to worry about the bill. This house is part of the Church and it's his Church now."
The new pontiff has postponed for a few days a trip to the papal summer retreat south of Rome, to meet Benedict, who last month became the first pontiff in 600 years to step down, saying that at 85 he was too frail to lead the troubled Church.
Francis is, at 76, older than many other contenders for the papacy and his age was one of several big surprises about the selection of the Argentine cardinal. The Vatican said today he was "in very good shape" despite having a lung partially removed more than 50 years ago.
French Cardinal Jean-Pierre Ricard told reporters: "We were looking for a pope who was spiritual, a shepherd. I think with Cardinal Bergoglio, we have this kind of person. He is also a man of great intellectual character who I believe is also a man of governance."