Update:Follow up post with some numbers and logic laid out. Muslims more numerous than Catholics: Vatican:
Monsignor Vittorio Formenti, who compiled the Vatican's newly-released 2008 yearbook of statistics, said Muslims made up 19.2 percent of the world's population and Catholics 17.4 percent. "For the first time in history we are no longer at the top: the Muslims have overtaken us," Formenti told Vatican newspaper L'Osservatore Romano in an interview, saying the data referred to 2006.
I'm willing to bet that Formenti is wrong. What do others think? Consider that Catholicism was the dominant religion in 1250 across 2/3 of Europe, and the residual Christian population in the Islamic world did not tend to be in communion with Rome (Maronites being the exception to the rule). I suspect that there were more Muslims than Roman Catholics until about the 18th century, when Islamicization of marginal peoples such as the Central Asian Turks was just about tapped out and population growth in the New World was adding to the numbers of Catholics. Population growth in Catholic European nations in the 19th and early 20th centuries would likely have increased the lead I'll try and look up some population numbers and post them. I know this is a pedantic point,
but when it comes to controversial topics which have public policy implications people should try really hard not to make stuff up when there's no need....