MADRID

 Formally welcomed to World Youth Day by a boisterous, flag-waving throng of hundreds of thousands of young people from around the world, Pope Benedict XVI encouraged their enthusiasm but also urged them to be strong, solid and think about their faith.

Pope Benedict walked through the Puerta de Alcala, a monumental arch symbolizing the entrance to the city, with young people representing Europe, the Americas, Africa, Asia and Oceania.

Moving to the nearby Plaza de Cibeles for the formal greetings and a prayer service, young people representing the various regions greeted the pope and gave him a gift that represented a formal cultural welcome. The pope received salt and bread from a young Polish woman; a flower garland from a Japanese woman; a bowl of rice from a South Korean; a sombrero from a Honduran; and coffee beans in a banana leaf from a young man from Australia.

Cardinal Antonio Rouco Varela of Madrid also took a turn at the microphone, welcoming the pope on behalf of the Spanish church and society, but the evening ceremony and prayer service Aug. 18 were clearly about the pope and the young people.

Hours before the pope arrived, young people staked out spots in the plaza and surrounding streets. To pass the time, they danced in the streets, sang, clapped and waved their nations' flags. Hundreds of thousands of people swayed to the beat of the "Macarena."

German Sarah Wang could hardly contain her excitement.

"You always see him on TV or in pictures; it's so exciting that he's actually in the same country (as me)," she said. "The last two days you are waiting for him, but now it's so different. That's the purpose why you're here, to see the pope and hear him.

"When you see the pope, you feel like you're Catholic," said Florence Pua, part of the Chinese-Filipino community in Manila, Philippines. "I want to see him so I can firm up my faith and detach myself from the things that are earthly."

The pope greeted the youths in Spanish, French, English, German, Italian, Portuguese and Polish.

In English, he expressed his hopes that "these days of prayer, friendship and celebration" would "bring us closer to each other and to the Lord Jesus. Make trust in Christ's word the foundation of your lives."

After the Gospel was sung in English, the pope gave the youths an in-depth introduction to the World Youth Day theme, "Rooted and Built up in Jesus Christ, Firm in the Faith."

The pope said that some words simply amuse or inform, but the words of Jesus "must reach our hearts, take root and bloom there all our lives."

He asked the young to listen to God's word and allow it to become "a rule of life which likens us -- poor in spirit, thirsting for justice, merciful, pure in heart, lovers of peace -- to the person of Christ."

World Youth Day is an opportunity to know Christ better and "to make sure that, rooted in him, your enthusiasm and happiness, your desire to go further, to reach the heights, even God himself, always hold a sure future, because the fullness of life has already been placed within you," he said.

The pope asked the young to be steadfast in faith, but also know that "in the face of our weaknesses which sometimes overwhelm us, we can rely on the mercy of the Lord who is always ready to help us again and who offers us pardon in the sacrament of penance."

He said some people "take it upon themselves to decide what is true or not, what is good and evil, what is just and unjust; who should live and who can be sacrificed in the interests of other preferences."

Such people claim to be living a life free from every constraint, but their lives have no mooring and no clear horizon, he said: They are lost.

The 84-year-old pope urged the young people to be "prudent and wise, build your lives upon the firm foundation which is Christ" so that "nothing will make you fear, and peace will reign in your hearts."

"Then you will be blessed and happy and your happiness will influence others," he said. "They will wonder what the secret of your life is" and they, too, will discover Christ, "your friend, brother and Lord, the Son of God incarnate, who gives meaning to all the universe."