Even though Indonesia boasts the largest Muslim population of any nation, it witnessed, in marked contrast to Egypt, a steady growth in the size of the Christian community in the course of the twentieth century. The Roman Catholic community grew from only 26,000 in 1900 to 500,000 in 1940, and to 6 million in 2003. The number of indigenous Protestants rose from 285,000 in 1900 to 1.7 million in 1940, and to perhaps 16 million in 2003. What is more, it is estimated that 1 million of the new Christians converted in the course of the century were of a Muslim rather than a traditional religious background.
That is all from the new and interesting Brian Stanley, Christianity in the Twentieth Century: A World History, published by Princeton University Press.