The basis of our entire belief starts with a simple statement لَا إِلَّهَ (There is NO إِلَّه). The reason I didn't define that word إِلَّهَ is because it doesn't just mean God. إِلَّهَ means someone or something that is loved, adored, worshipped and obeyed.
The reason the true definition is important because we are not muslim until we have completely and utterly testified and denied everything that we love and worship and obey before we come to Islam. If before Islam someone loved alcohol or gambling or any other sins that are against Islam, then they have to deny those desires before accepting Allah.
We as Muslims cling to our desires, our sins and still claim that we are Muslims. We haven't denied our sinful desires, we haven't left our God that we worship other than Allah.
How can we call ourselves Muslims while we still worship an إِلَّهَ other than Allah?