In helping those who are suffering, both young believers and nonbelievers can find "a strength that can change the world," the Pope said in a video message to youths for the upcoming World Youth Day in Panama.

"It is a revolution that can overturn the powerful forces at work in our world. It is the 'revolution' of service," he said in the message released by the Vatican Nov. 21.

The theme for the World Youth Day celebrations, which will take place Jan. 22-27, is taken from the Gospel of St. Luke, "May it be done to me according to your word."

In his message, the Pope said those words uttered by Mary during the Annunciation are "the positive reply of one who understands the secret of vocation: to go beyond oneself and place oneself at the service of others."

Life, he said, can only find meaning when serving God and others. Like Mary, young people must engage "in conversation with God with an attitude of listening" so that they may discover their calling either in marriage, consecrated life or the priesthood.

"The important thing is to discover what God wants from us and to be brave enough to say 'yes,'" the Pope said. "When God has a proposition for us, like the one he had for Mary, it is not intended to extinguish our dreams, but to ignite our aspirations."

Pope Francis encouraged young people to say 'yes' to God's calling, which is "the first step toward being happy and toward making many people happy."

"Dear young people," the Pope said, "take courage, enter within yourselves and ask God: 'What do you want from me?' Allow God to answer you. Then you will see how your life is transformed and filled with joy."