"For the first time in history, we are no longer at the top: Muslims have overtaken us," Monsignor Vittorio Formenti said in an interview with the Vatican newspaper L'Osservatore Romano. Formenti compiles the Vatican's yearbook.
Catholics are holding steady at about 17.4 percent of the world population, he said.
Muslims are now at 19.2 percent, according to data from 2006, he added.
(Related: "Islam Expanding Globally, Adapting Locally" [October 24, 2003].)
"It is true that while Muslim families, as is well known, continue to make a lot of children, Christian ones on the contrary tend to have fewer and fewer," the monsignor said.
Muslim countries compiled the figures on Muslims and then provided them to the United Nations, he said, adding that the Vatican could vouch only for its own data.
Christians overall—including Catholic Christians—make up 33 percent of the world population, Formenti said.
Spokespeople for the Vatican and the United Nations did not immediately return phone calls seeking comment Sunday.