I think, biggest difference is, that Islam is the only religion, where we have one God.
Um, nope.
The Holy Trinity may confuse some people and make them think that Chirstians serve three gods, or maybe one sometimes, or something silly like that. But it works like this: Jesus, The Holy Spirit, and God the Father (Body, Spirit, and Mind) are all one piece of a greater whole. Think of each piece as a body part: the arm isn't a leg, and the leg isn't a head; they are all indeed separate and distinct parts, but together they make up a complete body that is one.
