According to Wikipedia, the population is almost entirely Christian. One percent are Muslims, and they have apparently entered from neighboring countries.
Given what has happened to other African countries, invaded by Muslims coming down from the North, this would seem to be a smart move.
http://en.wikipedia.org/wiki/Angola#Religion
Religions:
indigenous beliefs 47%, Roman Catholic 38%, Protestant 15% (1998 est.)
Per CIA world fact book.
Witch Doctors and fetishes make up most of the indigenous beliefs.