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The challenge is to have Ethiopian Orthodox Christianity spread into Central Africa (OTL South Sudan, CAR, the Congos, etc.) because of Ethiopian missionaries or some other form of conversion. The POD must be before the 1880s (when Europe colonized Africa and brought Protestantism and Catholicism to the continent). If you want to do it before the creation of Islam it would also be interesting to see it spread into OTL Somalia or Yemen, but that would be harder.