Here's an online article about the Gospel roots of rap:

"Music historians are at last beginning to acknowledge that a key factor in the development of rap music were the early "straining preachers", many of whom recorded to extraordinary popularity in the 1920s and '30s. Compilation CDs such as 'The Roots Of Rap' (Yazoo) and 'Sacred Roots Of The Blues' (Bluebird) demonstrate that the raspingly rhythmic exhortations of preachers such as Rev F W McGee and Rev A W Nix were an early prototype for the secular rhymebusters of the 1970s. Of all the black preachers who were unexpectedly elevated to recording star status in the 1920s, the most popular was the Rev J M Gates. Born in 1884 the good reverend was the pastor of Mount Calvary Baptist Church in Rock Dale Park, Atlanta from 1914 until his death around 1941. His recording career began in 1926 when Columbia Records, intrigued by the novelty of a singing preacher (Rev Gates often sang gritty renditions of the old hymns accompanied by two or three uncredited members of his congregation) released the first of many subsequent releases. His first smash hit was "Death's Black Train Is Coming", a hair-raising, fire and brimstone sermonette.."

http://www.crossrhythms.co.uk/articles/music/Gospel_Roots/11534/p1/