To fight this, the pontiff argued, the U.S. church must study contemporary culture to find a way to appeal to youths. He made his remarks to bishops from Indianapolis, Chicago and Milwaukee who were making a periodic visit to the Vatican.
The American church "is called to respond to the profound religious needs and aspirations of a society increasingly in danger of forgetting its spiritual roots and yielding to a purely materialistic and soulless vision of the world," John Paul said.
"Taking up this challenge, however, will require a realistic and comprehensive reading of the 'signs of the times,' in order to develop a persuasive presentation of the Catholic faith and prepare young people especially to dialogue with their contemporaries about the Christian message and its relevance to the building of a more just, humane and peaceful world."
John Paul added: "An effective proclamation of the Gospel in contemporary Western society will need to confront directly the widespread spirit of agnosticism and relativism which has cast doubt on reason's ability to know the truth, which alone satisfies the human heart's restless quest for meaning."
All bishops must visit the Vatican every five years. This year is the Americans' turn, and several regional delegations already have met with the pope.
The pontiff's message comes after several years of turmoil for the Catholic church over priests' sexual abuse of minors, and church administrators' failure to sanction known offenders.
In American churches, several bishops have spurred controversy by suggesting that Catholic public officials — in particular,and New Jersey Gov. Jim McGreevey — should be denied communion for supporting abortion rights.
Other bishops have said that congregants might be barred from communion for voting for candidates who support abortion rights, stem-cells research, euthanasia or same-sex marriage.
A Harris poll conducted in the fall of 2003 found that 79 percent of Americans believe in God, while 12 percent aren't sure and 9 percent do not believe. However, only about a quarter of people attend religious services once a week.