A leading American Catholic theologian warned a St. Paul Church audience last night that “the European Union as we know it may cease to exist” in 10 to 20 years due to depopulation—a trend, he said, that results from a loss of faith.

The citizens of the E.U. are dying at a faster rate than they’re bearing children—with an annual rate of just 10.0 births per 1,000 people and 10.1 deaths, according to the CIA World Factbook.

While the U.S. population is still growing strong—1.71 American babies are born for every person who dies—George Weigel warned that Europe’s infertility is by no means an isolated phenomenon.

“Western Europe is playing the function of the canary in the cage in the mineshaft,” he said. “Are we all surrounded by air of a particularly toxic source?”

Weigel, author of the 1999 bestselling biography of Pope John Paul II, “Witness to Hope,” is a senior fellow at the Ethics and Public Policy Center, a self-described “Judeo-Christian” research institute in Washington, D.C.

His appearance kicked off the second annual lecture series sponsored by the St. Paul’s Lay Committee on Contemporary Spiritual-and-Public Concerns. The series will bring 15 speakers to the church, just steps away from Quincy and Adams Houses. The archbishop of Boston, Seán Patrick Cardinal O’Malley, began the event with a prayer.

Weigel brought star power—but not optimism—to the series’ kick-off.

“Why is Europe depopulating itself in numbers not since the Black Death?” Weigel asked the 200-strong crowd, composed of students and parishioners from Cambridge and Boston alike. “Why is it that in a few decades, Spain is anticipated to lose half of its population?”

The answer, Weigel said, lies with religion—or the lack of it.

“The 77-year-long civil war—1914 to the collapse of the U.S.S.R. in 1991— spilled oceans of blood and piled mountains of corpses, and left Europeans totally spiritually exhausted,” Weigel said.

That spiritual exhaustion accounts for the low fertility rates across the continent, he argued.

“In two decades...60 percent of Italians won’t have a brother, a sister, an aunt or an uncle,” Weigel said.

Words like “sibling” and “cousin” will become “purely dictionary” terms because “single children will marry other single children,” he said.

“None of the 25 countries in the European Union have a replacement birthrate,” Weigel said.

Despite that fact, the population of the E.U. countries is growing at an annual rate of 0.15 percent, owing to a stream of immigrants—largely from the Muslim world.

But Weigel seemed skeptical that the immigrants could be integrated easily into European society.

“One billion Muslims will not suddenly become good liberal citizens, whatever delusions they had three blocks away,” Weigel said, standing three blocks from Harvard Yard.

The Harvard-Radcliffe Catholic Student Association (CSA) put up flyers on campus advertising last night’s event. “We have an incredibly strong relationship with the parish,” said CSA President Michael V. Brewer ’07. “They provide a lot of material support, and we do what we can in return.”