Ask anyone the question: "Do you believe in God?" and you are sure to receive a variety of answers. The question should be "What do you believe about God?"
Think about this:
If there is a creation, there must be a Creator.
If there is a Creator, He must be the Sustainer.
The Creator Cannot Create Himself.
If He is the sole Creator/Sustainer -- He must be ONE.
God must be one. Otherwise we would see great differences and competition between the gods if there were more than one -- Alone.