Pope Benedict, wrapping up a triumphant return to his German homeland, this morning urged young people to shun a "do-it-yourself" concept of religion where they can choose what they want and disregard the rest.
A crowd of up to a million young people cheered the Pope at the last major event of his trip - an open-air mass outside the city to conclude the Catholic Church's World Youth Day.
Most had spent the chilly night in tents and sleeping bags, singing and praying in a festive atmosphere
Pilgrims stay on top of each other as they welcome Pope Benedict XVI at the Marienfeld, near Cologne, western Germany.
before the Pope arrived to say the Mass along with some 800 bishops.
Mostly in their teens and early 20s, the crowd rose to their feet when Benedict arrived in a popemobile to close the latest edition of the youth event started by his predecessor John Paul 20 years ago. They swayed and sang: "Jesus Christ, you are my life."
In his homily, read in five languages, Benedict used general principles to press his point, in contrast to John Paul who often spoke to youth specifically about sexual morality.
He asked them not to see religion as a "consumer product" where people choose only what they want from it and disregard rules that are sometimes difficult to observe.
"Religion constructed on a 'do-it-yourself' basis cannot ultimately help us," he said. "It may be comfortable but at times of crisis we are left to ourselves."
The impression left to the listener was that the new Pope was trying to preach without finger-wagging. "Freedom is not simply about enjoying life in total autonomy, but rather living by the measure of truth and goodness, so that we ourselves can become true and good," he said.
Asked what he felt about the Pope's words warning against pick-and-choose Catholicism, Malte Schubert, a 19-year-old German, said: "That means no sex, basically, doesn't it? He has to say that. He is the Pope, but I think people should make their own choices."
"I am part of a movement for devotion to the Virgin Mary. We strictly follow the teachings of the Church. The Pope is right to warn against do-it-yourself religion, it should be all or nothing," said Nuno Gonzago (20) from Portugal.
The Pope urged his listeners not to see Sunday Mass as an inconvenience and to build their dream for a better world by helping those less fortunate, particularly the sick and elderly.
The Pope, whose reserved style could have dampened the boisterous atmosphere of the festival that his charismatic predecessor dominated, has played his new public role without a slip and seemed to enjoy it at times.