Speaking to thousands of young people gathered in Rome on Sunday, Pope Francis talked about love and happiness, saying that love is the "Christian's identity card," and is "the only valid 'document' identifying us as Christians."

"If this card expires and is not constantly renewed, we stop being witnesses of the Master," he said in his homily during Mass in St. Peter's Square, attended by about 90,000 people, mostly teenagers, the Catholic News Service and EWTN report.

Departing from his prepared speech, the Pope said the "true friends" of Jesus stand out because theirs is a "genuine love that shines forth in their way of life" through "real actions."

"Those who are not real and genuine and who speak of love are like characters is a soap opera, some fake love story," he said.

The Pope said people at times may find difficulty in sharing or expressing their love for their fellowmen.

But he said there is a way to overcome that difficulty. "When loving seems hard, when it is difficult to say no to something wrong, look up at Jesus on the cross, embrace the cross and don't ever let go of his hand. He will point you ever higher, and pick you up whenever you fall," he said.

"Throughout life we will fall many times, because we are sinners, we are weak," the Pope said, again going off the cuff. "But there is always the hand of God who picks us up, who raises us up. Jesus wants us to be up on our feet!"

Pope Francis ended his homily by challenging young people to undertake "acts of great friendship and goodness" and living their "youth and all its gifts to the fullest and without fear of hard work."

"Be like sporting champions, who attain high goals by quiet daily effort and practice," he said. "Enthusiastically practice them, so as to be champions in life, champions of life! In this way you will be recognised as disciples of Jesus."

Pope Francis also told the youth that happiness is not a game app that they can download in their mobile phones in his homily for the Jubilee for boys and girls.

"Your happiness has no price," he said. "It cannot be bought. It is not an app that you can download on your phones nor will the latest update bring you freedom and grandeur in love."

Freedom, the Pope said, does not come from things people possess or from doing whatever anyone wants. Rather, true freedom and happiness can only be found in the love of Jesus, Pope Francis said.

The Pope dared young people to not "be content with mediocrity," or believe those who equate importance with the sort of toughness shown by heroes in films, or by wearing the "latest fashions."

"Don't believe those who would distract you from the real treasure, which you are, by telling you that life is beautiful only if you have many possessions," he told them.

The Jubilee for boys and girls is the latest phase in the Holy Year of Mercy, which began on Dec. 8. The three-day event started on Saturday when Pope Francis himself heard outdoor confessions from young teenagers in the middle of St. Peter's Square. This was followed by a youth rally in Rome's Olympic Stadium.