Pope Francis called for courage from the faithful in a Mass with cardinals in the Sistine Chapel today.
He warned that believers trying to do God's work in the world without reference to the cross of Jesus would be "mundane".
A Church that does not confess Christ, he said, would become nothing more than a "compassionate NGO".
"What would happen would be like when children make sand castles and then it all falls down," he said.
The Mass was held on his first full day as Pope after being elected on Wednesday.
He told the cardinals who elected him: "I would like all of us … to have courage to walk in the presence of God, with the cross of God, and build the Church on the blood that Jesus Christ spilt on the cross."
A Church that had the courage to confess the glory of Jesus Christ crucified would "carry on", he added.