Islam is on its way to become the most practiced religion in Europe. In a new book published by the University of Leuven, “The Iris and the Crescent,” sociologist Felice Dassetto says that Muslims will comprise the majority of the population of Brussels by 2030. The title of the book refers to the yellow flower symbol of Brussels’ region and to the Islamic emblem: While the first is decaying, the second is growing.

Muslims now make up one-quarter of the population of the capital of the enlightened Europe and they are asking to use the empty churches for Islamic prayers. Since 2008, the top seven baby boys’ names in Brussels were Mohammed, Adam, Rayan, Ayoub, Mehdi, Amine and Hamza. Mohammed is also the most popular name for baby boys in Belgium’s second-largest city, Antwerp, where an estimated 40% of elementary school children are Muslim.

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Yet Belgium is not an isolate case. Rabbi David Rosen, a moderate voice in the Jewish establishment, has warned that Europe risks being “overrun” by Islam. According to a recent report of the US Pew Center, Islam is already “the fastest-growing religion in Europe,” where the number of Muslims has tripled over the past 30 years. One third of all European children will be born to Muslim families by 2025.

Islam is the most practiced religion in the United Kingdom. In London, more Muslims attend mosques on Friday than do Christians churches on Sunday. The Oude Kerk, the oldest church in the city of Amsterdam, where the kings of Holland were crowned, is now a museum. The only “church” in the largest Dutch city that is crowded is the church of Scientology, a six-story building in the thick of the city center. Only 7% of Dutch Catholics now go to Sunday Mass and 16% of children are baptized.

In Austria, which was 90% Catholic in the 20th Century, Islam will be the majority religion among Austrians aged under 15 by 2050. The French case also shows that the often exaggerated “Eurabia” threat is more a quality phenomenon of religious attendance than of demographic takeover. In France, there are now more Islamic mosques being build - and more frequently so - than Catholic churches, and there are more practicing Muslims than practicing Catholics in the country.

Overall, the total number of mosques in France has already doubled to more than 2,000 in the last 10 years. The best known French Islamic leader, Dalil Boubakeur, Rector of the Great Mosque of Paris, recently suggested that the total number of mosques should double yet again, to 4,000, to meet the growing demand.

Meanwhile, the Catholic Church in France had only 20 new churches built in the last 10 years, and formally closed more than 60 churches, many of which became mosques, according to research conducted by the French Catholic daily La Croix.

Princeton University’s Bernard Lewis once told the German daily Die Welt that “Europe will be Islamic by the end of the century.” At the time, Brussels’ political and cultural elites expressed outrage at the alarmist prediction. Yet if the current trends persist, Mr. Lewis may yet be proven right.