in Africa, there is the Church of Alexandria, which was established by St. Mark the Blessed Apostle around 40 a.d.. There was the Alexandrian Theological School, part of the ALexandrian University, and many blessed Fathers were in this school from around the christian World.
Many don't know, and it is a shame, that there are around 15 million coptic orthodox christians in EGypt alone.
The See of ALexandria extends to the North of Africa, where there are a small number of christians, to Sudan and the Church of Ethiopia is a daughter church to the church of Alexandria. These regions are have still christians.
Nubia is not Sudan, it is an area extending 200 Km in South Egypt and 300 km in North Sudan along the Nile.
Africa was never predominantly muslim at any point of time. I don't know from where the africa americans have got such an idea .
The muslims are basically concentrated in the North (Egypt, Morroco, ALgeria, Tunisia, Lybia) and South to the Great Africa Sahara in CHad, Niger, Muritania, Mali, Burkina Faso, Tanzania and in SUmalia.
The rest of muslims are scattered.
The good news is that around 6 million muslim in AFrica per year (according to AL-Jezeera) become christians every year.
As for christianity in the Arabian penninsula, there were Jews and Christians in Mekka and Madina and in Yemen. Yemen was a flourishing orthodox christian community. There were tribes who embraced either religion before Islam.
However, it is not clear whether it was the orthodox faith they followed or heresies.
For example, the uncle of Muhamed's first wife, Waraqaa, was an Arian christian. His title was the Bishop of Mekka, and apparently he was preparing Muhamed for this role.
He wrote with him the first chapters of Islam and they depicts Christ as a semi-devine or perfect human creature, in line with the Arian heresy.
There were banished Nestorian, Macdonian, and Maryists monks who did preach their doctrines to the tribes.
The problem is that Islam erradicated all other religions and all other resources which would have gave hints to the existing culture. The arabic community before Islam was not pagan in its entire history, but Islam burned all libraries and books which would document any other culture.
There was a church in Mekka, adjacent to the Kebba, and which used the walls of the Kabba for icons. It was taken down by Muhamed.
There is evidence that there were christians in Iran.
Iraq, Syria, Lebanon were all christian places.