After years of worrying about Europe's future, Cardinal Joseph Ratzinger of Germany put his hopes and fears on the record during a 2001 interview.

There had been hints. German journalist Peter Seewald noted an old quote in which Ratzinger said the church would be "reduced in its dimensions, it will be necessary to start again." Had the leader of Rome's Congregation for the Doctrine of the Faith changed his views?

"Statistical data shows irrefutable tendencies," replied Ratzinger. "The mass Church may be something lovely, but it is not necessarily the Church's only way of being.

"The Church of the first three centuries was small, without being, by this fact, a sectarian community. On the contrary, it was not closed in on itself, but felt a great responsibility in regard to the poor, the sick."

Four years later, this bookish cardinal became Pope Benedict XVI, serving until his stunning resignation in 2013 -- the first pope to resign in 600 years. Meanwhile, waves of change have continued to rock Eastern and Western Europe.

Now, the Benedict XVI Centre for Religion in Society, based at St. Mary's University in London, has released a study showing that Christianity is no longer Europe's default religion, especially among the 16- to 29-year-olds who are its future. "Europe's Young Adults and Religion," was produced with the Institut Catholique de Paris, analyzing data from 22 countries, drawn from the 2014-2016 European Social Survey.

In 18 of these countries "fewer than 10 percent of all 16-29 year-olds attend religious services at least weekly. And in 12 of them, over half say that they have 'no religion,' " noted Stephen Bullivant, the report's author and director of the Benedict XVI Centre, in email exchanges with Rod Dreher of The American Conservative.

"These are all countries in Europe, the very heart of Christendom, where Christianity (albeit in several forms) has been reliably passed on from generation-to-generation for the best part of 2000 years. And now, in the space of just a couple of generations, that's largely stopped in many places."

The key, he said is that "nominal" or "cultural" faith doesn't pass from one generation to another. While today's parents may call themselves Christians -- perhaps believing in a "vaguely benevolent 'Something' out there" -- their children have cut these ties to the past, said Bullivant, a former atheist who converted to Catholicism in 2008.

Among the report's key findings:

* The Czech Republic defines one side of this continuum, with 91 percent of its young adults saying they have no religious ties and 70 percent saying they never attend worship, other than "special occasions" -- funerals and weddings. As for prayer, 80 percent say "never." However, a quarter of young Czech Catholics attend Mass at least weekly, a potential sign of life.

* With a population that's 82 percent Catholic, Poland represents the other extreme. Among young Catholics, 42 percent say they attend Mass weekly and 50 percent say they pray -- outside of religious rites -- at least once a week. Only 17 percent of young Poles claim no ties to religious institutions, the same percentage as those who said they never pray.

* Even with its strong Catholic heritage, only 7 percent of young adults in the Netherlands claimed that faith -- the same percentage as in the Czech Republic. Among young Dutch Catholics, only 7 percent said they attend Mass weekly, while 43 percent still claimed to prayer at least once a week.

* In the UK, 70 percent of young adults claimed no traditional faith, along with 64 percent in historically Catholic France. Only 26 percent of young adults in France called themselves "Christians," while 21 percent did so in Britain. In France, 10 percent of young adults identified as Muslims, compared with 6 percent in Britain.

If the old Europe fades, the future Pope Benedict predicted that a smaller church would have to return to basics, with believers choosing to "be missionaries, in the sense of proposing again to society those values … which are at the base of the possibility to build a really human social community."

It will be crucial, he added in 2001, to "propose again the great underlying constants in their fundamental components, the questions on God, salvation, hope, life, especially what has a basic ethical value."