the origin of islam, and the people that spread it, is almost certainly as a heresy of the type of christianity practiced by people that lived in the kingdom of aksum, which is the geopolitical entity that dominated ethiopia, which again was christian during late antiquity and the early middle ages.

there were wars fought between aksum and rome and they probably took an unclear side in the long roman-persian wars, which allowed them to overrun the middle east at the conclusion of the catastrophic heraclean wars, and basically take over the area permanently.

this is obscured in the history but it is clarified by the genetics.

https://en.wikipedia.org/wiki/Kingdom_of_Aksum