Indonesia's population is about 262 million (USA = about 326 million). Nearly 90% of the people identify as Muslim. It boasts the largest Muslim population in the world. Yet, the growth in Christianity has been steady over the course of the twentieth century. Catholics grew from just 26,000 in 1900 to 500,000 in 1940, and to 6 million in 2003. Protestants increased from 285,000 in 1900 to 1.7 million in 1940, and to roughly 16 million in 2003.
Conversion from Islam is unusual but not freakishly rare.