COLOGNE, GERMANY ? Pope Benedict XVI triumphantly ended his four-day trip to his native Germany on Sunday, celebrating an open-air Mass for a million people and warning Europe against growing secularism and “do-it-yourself” religion.

A synagogue visit, in which he won applause for his warning about rising anti-Semitism, and a frank talk with Muslims about terrorism, underlined interfaith relations as a key theme of his first foreign travel as pope.

During his closing homily at Sunday Mass, Benedict told those gathered for the church’s 20th World Youth Day festival in Cologne, Germany, that there was a “strange forgetfulness of God,” while at same time the sense of frustration and dissatisfaction has led to a “new explosion of religion.”

“I have no wish to discredit all the manifestations of this phenomenon,” he said. “Yet, if it is pushed too far, religion becomes almost a consumer product. People choose what they like, and some are even able to make a profit from it.”

“But religion constructed on a ‘do-it-yourself’ basis cannot ultimately help us,” he said.

The throngs from almost 200 countries had been invited to the festival by a different pope, the charismatic John Paul II, before his death April 2.

But they embraced his 78-year-old, more subdued successor with the same huge turnout, shouts and applause on his first foreign trip as pope.

In this picture released by Vatican newspaper L'Osservatore Romano, Pope Benedict XVI waves to faithful Sunday as he arrives for the concluding Mass of the World Youth Day at the Marienfeld near Cologne, Germany. The Pontiff celebrated an open-air Mass before more than 1 million young people Sunday, calling on the pilgrims to make wise use of the freedom God has given them.

“Beeen-e-DET-to, Beeen-e-DET-to,” they chanted, using the Italian version of his name. Some 800,000 of them spent the night in the Marienfeld, or Mary’s Field, outside Cologne, sleeping on the ground so they could attend Sunday’s Mass. Flags from dozens of countries floated over the crowd.

Benedict did not remind his youthful audience of the Roman Catholic Church’s bans on premarital sex and the use of condoms and other forms of artificial birth control, favorite topics of John Paul.

And he made no promise to attend the next World Youth Day in Sydney, Australia, in 2008; John Paul would always end World Youth Day – which he founded in 1984 – by saying he would come to the next one.

He urged people not to forget Sunday Mass when they arrived back in their home countries: “If you make the effort, you will realize that this is what gives a proper focus to your free time.”