Oh this is fun... just let it trickle over you a few times - I think you'll like it too:
In my church daze I learned that "true believers" made this sort of claim and it went right into how there is only one god but Muslims and others do not know that yet. Of course, if you were Muslim you saw it from your point of view and the other religions were wrong.
Just like 'Christianity' there are numerous sects and those sects often differ greatly from one another. Some of the most vicious fighting in the world is between the Sunnis and Shia sects of Islam.