Pope Benedict XVI urged 350,000 young pilgrims to become prophets of a new age bringing renewed faith to a spiritually barren world.
The Pope's ringing challenge echoed over a vast sea of Catholics packing Sydney's Royal Randwick Racecourse at the concluding mass of week-long World Youth Day (WYD) celebrations, which at times attracted up to half a million pilgrims and well-wishers.
His message capped a triumphant first trip to Australia for the 81-year-old pontiff, who flies back to Rome on Monday after a landmark visit that included a papal apology to victims of church sexual abuse.
Declaring the spirit of the church alive and well, the Pope told pilgrims from more than 170 countries he had shared an "unforgettable experience" in the great south land.
"Our eyes have been opened to see the world around us as it truly is, 'charged' as the poet says, 'with the grandeur of God', filled with the glory of His creative love," he said.
Pope Benedict said a new generation of Christians was being called to help build a world in which God's gift of life was welcomed and love was not greedy or self-seeking but pure, faithful and genuinely free.
He spoke of a "new age in which hope liberates us from the shallowness, apathy and self-absorption which deaden our souls and poison our relationships".
"The world needs this renewal," he said in a homily beamed to hundreds of millions of television viewers worldwide.
"In so many of our societies, side by side with material prosperity, a spiritual desert is spreading: an interior emptiness, an unnamed fear, a quiet sense of despair.
"How many of our contemporaries have built broken cisterns in desperate search for meaning - the ultimate meaning that only love can give?"
The church also needed this renewal, he said, adding: "She needs your faith, your idealism and your generosity so that she can always be young in the spirit."
"Do not be afraid to say yes to Jesus, to find your joy in doing his will, giving yourself completely to the pursuit of holiness, and using all your talents in the service of others."
The pontiff was speaking from a sanctuary 25m off the ground and painted deep red to reflect the Australian outback and the flames of the holy spirit.
His audience included 26 cardinals and 420 bishops at a grand mass featuring a 300-strong choir and an 80-piece orchestra.
The Pope had a bird's eye view of the faithful when he flew over Randwick in a helicopter on Sunday morning before doing a lap of the racecourse in his Popemobile.
At the mass he officiated over the sacrament of confirmation for 24 adults, including two from each Australian state.
Most pilgrims at the mass, an estimated 235,000, had camped out under the stars on Saturday night after making a nine kilometre pilgrimage across the Sydney Harbour Bridge and through city streets.
The pilgrimage was one of a number of WYD events that brought Sydney to a standstill, including the spectacular Stations of the Cross re-enactment of Christ's final days and the Pope's official arrival via a Sydney Harbour "boat-a-cade".
Australia's Catholic leader, Sydney Archbishop George Pell, told pilgrims the church was too often weighed down with the sins of her children, too often appeared disfigured and discouraged.
But at World Youth Day, the church "appears as she truly is, alive with evangelical energy".
Cardinal Pell described WYD as a gift for both young and the old.
It was a message eagerly taken up by Mary Galea, 69, from Melbourne, who said: "I'm really glad that God let me live so long I could experience this. I took communion and it was the most wonderful experience of my life."