"I am sure that you don't want to be duped by a false freedom, always at the beck and call of momentary fashions and fads," he added.
Pope Francis encouraged them to be "aiming high," resolving to maintain the lofty standards of truth, goodness, and beauty.
The Bishop of Rome's next image considered the field of faith as an athletic training ground, saying that being a disciple of Christ is much like joining a soccer team. Both require discipline and training.
He referred to St. Paul's writing that "athletes deny themselves all sorts of things" in order to "win a crown of leaves that withers," and then added that "Jesus offers us something bigger than the World Cup," soccer's international tournament which is to be held next summer in Brazil.
Christ offers "the possibility of a fulfilled and fruitful life; he also offers us a future with him, an endless future, eternal life," Pope Francis preached.
"But he asks us to train, 'to get in shape,' so that we can face every situation in life undaunted, bearing witness to our faith."
Such spiritual exercises, the Pope said, include talking with him in prayer, "our daily conversation with God." Do we really seek time in conversation with God, he asked, calling prayer also a "dialogue" with God.
Other spiritual exercises Pope Francis mentioned were the sacraments and loving one another, "yearning to listen, to understand, to forgive, to be accepting and to help others, everybody, with no one excluded or ostracized."
Finally, Pope Francis spoke of a field as a construction site, saying God constructs the Church of "living stones," and that "we are never alone" as we journey "on the same path."
"Jesus is asking us to build up his Church, but not as a little chapel which holds only a small group of persons. He asks us to make his living Church so large that it can hold all of humanity, that it can be a home for everyone!"
Christ says "to me, to you, to each of us," Pope Francis reflected: "Go and make disciples of all nations."
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"Tonight, let us answer him: Yes, I too want to be a living stone; together we want to build up the Church of Jesus!"
The desire, so characteristic of young people, to build up a more just, fraternal society, Pope Francis said, must begin with each person becoming more just and fraternal.
Dear friends, never forget that you are the field of faith! You are Christ's athletes! You are called to build a more beautiful Church and a better world."
Mary is the model for building a beautiful Church and better world, he assured the pilgrims as he concluded.
"All together, let us join Mary in saying to God: let it be done to me as you say. Amen!"