Pope Benedict has ended his first trip abroad, during which he celebrated World Youth Day. He also visited a synagogue, and reached out to Muslims and members of other Christian Churches.

Pope Benedict's last meeting in Cologne was with Germany's bishops. The pope told them that, in spite of increasing secularism and what he called de-christianization, they should remain united and persevere in their mission.

At the airport, before returning to the Vatican at the end of a four-day visit, the pope thanked all those who opened their homes to the young pilgrims who came to Germany from around the globe to celebrate the 20th World Youth Day.

"Now," he said, "young people can return home enriched by their contacts and their experiences of dialogue and fellowship. During these days," the pope said, "Germany has been the center of the Catholic world."

Pope Benedict came to Germany to celebrate an event that was originally planned by his predecessor. The pope mentioned Pope John Paul on numerous occasions, and blessed a bell in his memory during an evening prayer vigil on Saturday.

For the hundreds-of-thousands of young people who came to see the pope, it was a wonderful experience. But the pope did not always look at ease with the huge crowd that assembled to hear him celebrate Mass Sunday morning, and more than once, he asked them to be quiet.

In his message to the youth, he urged them to make time to attend Sunday Mass. He also warned them against what he called "do-it yourself" religion. But while youngsters here listened to the pope carefully, many said they would continue to make their own choices.

At the end of the huge open-air mass Sunday, the pope announced that the next World Youth Day would be held in Sydney, Australia, in 2008.

Pope Benedict used his visit to his native Germany to reach out to non-Catholics. He became the second pope to visit a synagogue, and told Jewish leaders that he intended to continue to improve relations between Jews and Catholics. He warned of rising anti-semitism.

In a meeting with the city's Muslim community of Turkish origin, he addressed the issue of terrorism. He said joint efforts must be made to turn back the tide of what he called this "cruel fanaticism." He also told Muslim leaders they have a responsibility to educate their young followers against violence

The pope also reached out to members of other Christian denominations, stressing the Catholic Church's goal of full and visible unity.