Francis remarked on the "festive mood" of the event in Poland, and with all the young people taking part both in person and via modern media, "we are going to make this World Youth Day an authentic Jubilee celebration."
"Nothing is more beautiful than seeing the enthusiasm, dedication, zeal and energy with which so many young people live their lives," he said.
"When Jesus touches a young person's heart, he or she becomes capable of truly great things."
The Roman Pontiff described his excitement in listening to the young people share their dreams, their questions, and "impatience with those who say that things cannot change."
"It is beautiful and heartwarming to see all that restlessness!" he said. "Today the Church looks to you and wants to learn from you, to be reassured that the Father's Mercy has an ever-youthful face, and constantly invites us to be part of his Kingdom."
"Knowing your enthusiasm for mission, I repeat: mercy always has a youthful face!"
A merciful heart is one which seeks to go beyond its comfort zone, to go out and embrace everyone, the Roman Pontiff continued.
It is also a "place of refuge" for the homeless, refugees, and migrants; "it knows the meaning of tenderness and compassion."
"To say the word 'mercy' along with you is to speak of opportunity, future, commitment, trust, openness, hospitality, compassion and dreams."
The Pope then spoke off the cuff, saying that "when the heart is open, it is able to dream; there is a room for mercy, there is room to caress those who suffer."
"There is room to place oneself next to those who lack peace in their heart, or who lack the necessities of life, or who lack the most beautiful thing: the faith."
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Francis discouraged young people from entering into an "early retirement" – referring particularly to those "who are defeated even before they begin to play, who walk around glumly as if life has no meaning."
"Deep down, young people like this are bored… and boring!" he said.
The Pope also said he is troubled when young people "waste their lives looking for thrills or a feeling of being alive by taking dark paths and in the end having to pay for it… and pay dearly."
"It is disturbing to see young people squandering some of the best years of their lives, wasting their energies running after peddlers of fond illusions."
He challenged young people with an alternative: to seek the "power of grace" which gives them a "lasting sense of life and fulfillment."
"To find fulfillment, to gain new strength, there is a way. It is not a thing or an object, but a person, and he is alive. His name is Jesus Christ."