Why didn’t Christianity spread southwards from Ethiopia in the Middle Ages?

I suspect that after the king of kings of Aksum converted to Christianity the government gradually converted all the people to Christianity. And whenever Aksum conquered a new region efforts would have been made to convert the population. One area where Aksum expanded was into South Arabia. But if Aksum converted a lot of Arabs to Christianity the effort was wasted when the Muslims conquered all of Arabia about AD 630, and began converting everyone to Islam.

The Muslims conquered Egypt a few decades later and began expanding south, but were blocked by the Christian states in the Sudan such as Makuria. But Muslim traders introduced Islam and the country gradually converted to Islam by about the 14th century or so. Then any Ethiopian efforts to expand west into the Nile valley would have been strongly resisted by the mainly Muslim population.

Meanwhile Arab traders established many trading posts and cities along the east coast of Africa and naturally introduced Islam, which spread from those communities rapidly or slowly in various cases.

So by then the Ethiopians would have had to expand directly south avoiding the Muslim realms in the Nile valley to the west and the Muslim lands along the African coast to the east, or else having to risk fighting fierce Muslim opposition to Christian conquests.

So part of the reason why Ethiopia didn't convert more southerly regions of Africa was that Ethiopia didn't conquer more southerly regions of Africa. Ethiopia had a window of opportunity of about 500 or 1,000 years when it could theoretically have conquered large regions of Africa and formed a vast empire and converted most of the inhabitants to Christianity, but didn't use that opportunity.

So some might say that Ethiopia wasted that window of opportunity to conquer and convert vast areas of Africa. But other persons might suppose that Ethiopia wasn't a military superpower for most or all of those 1,000 years and didn't have the military and logistical capability to conquer much larger areas that Ethiopia did sometimes conquer during periods of Ethiopian strength.

And it is possible that Ethiopian traders and missionaries did convert many Africans in distant regions, and that the later success of Muslims in converting Africans to Islam might have been made easier by some people in those regions already being Christian or part Christian.

There wasn't anything for it to spread south to.

OK, there is one exception that Mr. de Bernardy pointed out in the comments. Somalia is south of Ethiopia (when it wasn't part of its empire), and there were conversions there. Many people living there were Jewish and Christian, and some of the Jewish converts may have reached as far south as modern Tanzania. After Mohammed, most of the folks in Somalia converted to Islam, and stayed with that religon throughout the middle ages. Of course this region had strong trading and cultural contacts with the Middle East. So I think Somalia may be useful as the exception that proves the rule.

Christianity is what is often called a religion of the book. It is a scriptural religion, which means it really is designed for use within a literate society. That isn't to say that illiterate people can't follow it, but someone in the society needs to be able to read from scriptures.

Additionally, converting to a trans-national religion isn't all about belief. More fundamentally, its a good way to acknowledge existing trade and cultural ties with another country/state/region, and to actively strengthen them. If no such ties exist, and the country's people and rulers don't want them, there's no real use to them for that country's religion.

Somalia aside, there were no other literate societies in Africa south of Ethiopia in the Middle Ages. We don't even know of many states at all south of there. There was the Bachwezi in the Great Lakes area around 1300, and Great Zimbabwe in South Africa from around 1100, but both appear to have been illiterate societies, the later with no known external contacts with any other society. The other known pre-colonial native African states south of there (Lunda and Luba) were founded after the 1400's, and their contacts north would have likely been with Islamic societies.

_{Map showing pre-colonial African kingdoms} ^{Larger version}