When I think of African Christianity only Egypt and Ethiopia come to mind. Would it be fair to say that the African slave trade was the first contact most of those Africans had with the Christian religion?
This is my first time posting here so please be gentle!
To answer your question it might be easier to qualify during what particular time period.
Eusebius^[1] a Christian Historian writing in the 4th century wrote that Mark (one of Jesus's Disciples) came to Egypt (Alexandria) in 41-43AD. Spreading to Sudan by the end of the 1st Century. In the year 189 we have our first Pope from the continent of Africa from Leptis Magna (Modern day Libya) so we get a good impression that Christianity spread relatively quickly through Roman Africa.
By the 5th Century all of Roman Africa (Northern sections of Algeria, Libya, Tunisia, Egypt and portions of the Maghreb region including Morocco) was effectively Christian. With Ethiopia officially converting as a nation in the 4th Century under King Ezana of Axum and a sizeable Christian population in Nubia (southern Egypt/Northern Sudan) around the same time.
Arab Invasions (notably that of Egypt in the 7th century) wiped Christianity for the most part off the face of North Africa. But the major exception to this rule was the Nubian kingdom of Makuria and Ethiopia.
Makuria which whilst being cut off from the rest of Christendom by Arab advances remained independent and Christian - signing a peace treaty with the Arabs allowing trade in the 7th century which lasted until the 1300's where it eventually collapsed in the face of Egyptian aggression.
Ethiopia ultimately was the only nation to collectively resist Islam during its spread throughout Africa partly thanks to support from European monarchies.
Some Christians continued to exist in conquered North African countries, the most successful being the Egyptian Copts. It wasn't until the French arrived in places like Algeria and Tunisia that Christianity was reintroduced in the 1800's. Christianity later emerged again under European colonies being established (Cape Town etc. in 1652) and the emergence of the European Slave Trade which opened up much of Subsaharan Africa to missionaries.
So in closing in North Africa Christianity at one point was very common and was effectively driven close to extinction but maintained some presence right up until today. However it was introduced (or reintroduced in some places) with the advent of European trade and slavery in 17th Century Subsaharan Africa.
Albeit its worth noting that large portions of Africa remained unreached until recently. Only nine million Christians were in Africa in 1900, but by the year 2000, there were an estimated 380 million Christians. Much of this growth and encounter with Christianity has come from a mixture of European, American and Native peoples mission work. The majority of this is in Subsaharan Africa as opposed to North Africa. Also its worth considering countries like Liberia which was founded with the idea of resettling slaves from the US, the majority of which carried their Christian faith back to Africa.
Conclusion The honest answer is that most people in Africa who encountered Christianity for a long time have only encountered it in the last 100 years or so. Whilst Colonialism was a facet of its emergence back in Africa after its suppression under Islam many more Africans have encountered Christianity in a post colonial era than a pre colonial era. The Colonial period itself wasn't when most Africans who have encountered Christianity encountered it.
Sources Of Interest
Church History, Eusebius, 4th Century AD
The last native Christian communities of North Africa - http://rhr.revues.org/5401
Indigenous Christian Communities in Islamic Lands, M. Gervers and R. Bikhazi, Toronto, 1990.
Ecclesiastical Cartography and the Invisible Continent: The Dictionary of African Christian Biography, J. Bonk, 2008 - http://www.dacb.org/xnmaps.html
Christian Nubia at the Height of its Civilization. UNESCO General History of Africa, Volume III. University of California Press, S. Jakobielski, 1992
P.S This is my first time posting in this sub so please forgive me if I've gotten anything wrong in my submission. Its a massive subject and I've given my best shot to give a brief overview. I'm sure other good folk exist on this sub who could be more comprehensive.
The main I would add on regarding West Africa is that Islam started to spread in that area around the 8th century AD. A chief reason that Christianity didn't spread to West Africa in comparison to East Africa lies in the fact that North African Christianity was predominantly urban