I've just stumbled across the Pew Forum's extensive report on Islam and Christianity in Sub-Saharan Africa. It reveals the incredible transformation of the region over the last century. According to the report:
In little more than a century, the religious landscape of sub-Saharan Africa has changed dramatically. As of 1900, both Muslims and Christians were relatively small minorities in the region. The vast majority of people practiced traditional African religions, while adherents of Christianity and Islam combined made up less than a quarter of the population.
Since then, however, the number of Muslims living between the Sahara Desert and the Cape of Good Hope has increased more than 20-fold, rising from an estimated 11 million in 1900 to approximately 234 million in 2010. The number of Christians has grown even faster, soaring almost 70-fold from about 7 million to 470 million. Sub-Saharan Africa now is home to about one-in-five of all the Christians in the world (21%) and more than one-in-seven of the world's Muslims (15%).
Since 1910 Christianity has been outgrowing Islam in Sub-Saharan Africa by a rate of 3.5 to 1.
This map reveals a continent divided between Islam and Christianity.