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.

Do we agree that this is a creation? Or do we accept that nothing came out of nothing to form this entire universe?
This is a very clear message from Allah, in the Quran. Something does not come out of nothing. So, there must be something in existence already which created all that we know to exist. And that "something" needs to be called upon in times of need and thanksgiving.
As Allah has said in His Book: "I only created you all to worship Me Alone."

He also says that we are all being tested by Him with regard to our wealth, families, children and social status.