Pope Benedict XVI urged young people Sunday to reject what he said was the "spiritual desert" spreading throughout the world and to embrace Christianity to build a new age free from greed and materialism. At a Mass before more than 200,000 young Roman Catholic pilgrims in Sydney, Benedict said "the world needs renewal" and challenged them to be the agents of change. Organizers said at least another 200,000 pilgrims gathered at other sites nearby to watch on giant television screens. "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," the pontiff said. The 81-year-old pope said it was up to a new generation of Christians to build a world in "which God's gift of life is welcomed, respected and cherished - not rejected, feared as a threat and destroyed." The aim was "a new age in which hope liberates us from the shallowness, apathy and self-absorption which deadens our souls and poisons our relationships," he said.