… A rough geography
What is sub-Saharan Africa? According to the UN, everything except Western Sahara, Morocco, Algeria, Tunisia, Libya, Egypt – and Sudan, too, though it is south of all these. But the statistics of UN institutions do normally count Sudan as sub-Saharan.
So Mauritania, Mali, Niger, Chad are considered sub-Saharan. If you add Sudan, they form another chain from the Atlantic to the Red Sea. They contain most of the Sahel.
All these countries are Muslim (or Moslem, as we used to say), but two of them, Egypt and Chad, have substantial Christian minorities.
Further south, Senegal, the Gambia, Guinea, Djibouti, Somalia are Muslim. Sierra Leone and Burkina Faso are predominantly Muslim, but have substantial Christian minorities of various denominations.
More evenly balanced are Guinea-Bissau, Côte d’Ivoire, Togo, Benin, Nigeria. Guinea-Bissau is half-Muslim, 10% Christian and the rest animist or traditional – but when we talk about traditional, we are sometimes including localised versions of Islam and Christianity, and when people declare as Muslims or Christians, they may still bring in elements of a local religion: figures or estimates for one country are not necessarily comparable with figures for another. Côte d’Ivoire is half-Muslim, half-Christian, apart from a small traditional element. Togo is 30% Christian, 20% Muslim, with the rest traditional. Benin is 43% Christian, 25% Muslim. Nigeria is roughly half-Muslim, half-Christian, with the Muslims in the north.
Liberia, Ghana, Cameroon, Central African Republic, Equatorial Guinea, Gabon, both Congos, Uganda, Kenya, Rwanda, Burundi, Angola, Zambia, Malawi, Namibia, Botswana, Zimbabwe, South Africa, Lesotho, Swaziland have large Christian majorities. (Malawi, in this group, has the largest Islamic minority. [Postscript: See comment below.])
Eritrea, South Sudan, Ethiopia, Tanzania, Mozambique have smaller Christian majorities. Eritrea has a large Islamic minority. South Sudan has a large traditional minority and a small Islamic one. Ethiopia and Tanzania have large Islamic minorities. Mozambique has a smaller Islamic minority, but still only a small Christian majority, according to official figures, with many undeclared. (Madagascar is half-Christian, with the rest mainly traditional; there is a small Islamic minority.)
Most African Islam is Sunni, but there are Shiite communities in Egypt, Senegal and Nigeria and Ismaili Shia communities, established by immigrants from South Asia, in East and Central Africa and in South Africa. Cairo, of course, though the population is overwhelmingly Sunni, was an Ismaili Shiite foundation.
The dominant churches in Egypt, Eritrea, Ethiopia are Oriental Orthodox. The Christians in Chad are Catholic and Protestant.
Christians outnumber Muslims in sub-Saharan Africa (including Sudan?) by two to one. Under half are Catholic. Many are renewalists (Pentecostals and Charismatics), Seventh-day Adventists and Jehovah’s Witnesses.