God is a creation of Man for the purpose of serving as an instrument of government to assist him in his politics, morality and spirituality.
It is quite possible to argue that none of the Prophets of God in fact believed in the actual existence of a living God, and only had the same motivation as parents who make their children believe in Santa. That motivation is to incentivize better behaviour than might otherwise be shown without a belief in Santa or God.
"Let us with caution indulge the supposition that morality can be maintained without religion." George Washington
The purpose of Judaism is to demonstrate the superiority of a people who created God and who do what their God commands, even if that God does not promise a heaven or threaten a hell in the afterlife. (If that is the case then I am at heart a Jew.)
It was the purpose of Christianity to spread the benefits of a unique and omnipotent God to those who are not born Jews and would not wish to convert. Unfortunately, they did this by means of insisting on their totalitarian doctrine that one must believe in the divinity of Christ to be a Christian, or else go to hell. (If that is the case then I am at heart a Christian who wishes non-Jews to enjoy the benefits of believing in an omnipotent perfect God that the Jews created for the benefit of Jews.)
The purpose of Islam is to synthesise the previous two religions by making it easier to practice and easier to understand through the means of one book said to be the direct revealed Word of God, which its adherents are meant to consult as often as they can. To be a Muslim, all one need do is to declare that one submits to God and affirm that Muhammad was His prophet.
If you understand the concept of a unique and omnipotent God, then you would have to accept the inexorable logic that there cannot be other gods but God, because, being omnipotent, He would not suffer them to exist to be His rivals.
God, if He exists, would embody Truth, Reason and Reality, all of which I am perfectly happy to submit to.
I have no difficulty agreeing with Muslims that Muhammad is a Prophet of Islam either.
http://en.wikipedia.org/wiki/Shahadah
There is no god but God, and Muhammad is the messenger of God.
