"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.
He said that Catholics accounted for 17.4 percent of the world population - a stable percentage - while Muslims were at 19.2 percent.
"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.
Formenti said that the data refer to 2006. The figures on Muslims were put together by Muslim countries and then provided to the United Nations, he said, adding that the Vatican could only vouch for its own data.
When considering all Christians and not just Catholics, Christians make up 33 percent of the world population, Formenti said.
Spokesmen for the Vatican and the United Nations did not immediately return phone calls seeking comment Sunday.