I have played on both sides of the tracks so I will try to explain a little. Rap is just a vocal style just like singing. Some of us love distortion on guitars some of us prefer acoustic. It is the same idea. It is just a style. Rap and Hip Hop did in the 80s exactly what Rock and Roll did in the 50s. It is a new way to express yourself.
The context is still basically the same in many cases. The best example of this is is Eminem's version of Aerosmith's Dream On. Eminem called it "Sing for the Moment". The young rapper heard the same classic song we all heard and felt he could say it in a way his generation could better understand. The song is about clawing your way to the top. It's about your hopes and dreams coming true because of your hard work.
When Joe Perry heard Eminem's words he asked to play on the new track. He knew what was being said and believed it kept the spirit of the song intact. Don't take my word for it listen yourself.
Christian rap expresses the same thing as Christian rock. Both of those came from Gospel, just with a little distortion on it.
If you like christian rap check out http://www.myspace.com/hoperecordz
My boy Mingo is on this label. Hot Hot MC that went back to his christian roots after we spent some time in the clubs (both Rock and Hip Hop). He played drums for me in a real hard band called Trip Society. I've played bass for some of his Hip Hop tracks. We all trying to express the same thing.
One of my (many) new projects is playing bass in a traditional Gospel band (they know nothing about Rap). I can't wait for the opportunity to bring Mingo into this project. It is all music. We all play it and we all trying to express the same thing.