Paul Bowers wrote an article more than 25 years ago that spoke of the deep roots of Nubian Christianity.  In his article, "Nubian Christianity: The Neglected Heritage," Bowers asserts that there was a lively Christianity in northern Sudan that lasted more than a millennia. Why it survived as long as it did, but also its demise in the 15th century is the subject of this paper which he wrote back in 1985 for the Africa Journal of Evangelical Theology iv.1 (1985) 3-23. It is an article worth reading and pondering. We provide the link below:

http://www.theoledafrica.org/OtherMaterials/Files/NubiaNeglectedHeritage.pdf