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Dr Abdullah Hakim Quick, explains on the Deen Show http://www.TheDeenShow.com with Eddie, that Jesus and all the prophets from Adam to Muhammad (peace and blessings be upon all of them) are Muslims.
Christianity is a term people derived from the name (Christ) as explained in greater detail by Ahmad Deedat in this video: http://www.youtube.com/watch?v=bHFKFSk4qY0