Just because Christianity and Islam are rooted from the God of Abraham does not mean they are the same God. Christianity and Islam were established to replace the former belief system and were never meant to be synchronous with other religions.
What would be the point of trying to get people to convert if one or the other is just as good? Yet, the sense of urgency to create converts is at the very heart of both Christianity and Islam. There also lay one of the main differences in how each religion approaches conversion/evangelism. Christianity uses baptism by water, Islam uses the sword.
The second, kind of a baptism in evil.
Islam is from the devil.