“Is there a God beside Me? Yea, there is no God; I know not any”, God says (Isa. 44: 8);
“The Lord He is God; there is none else beside Him” (Deut. 4:35).
Such verses occur time and again throughout the Bible. Because God is the source of all power and the only God, He is therefore a jealous God, as He often reminds us (e.g. Ex. 20:5; Deut. 4:24).
God gets jealous when His people start believing in other gods, if they say to Him, ‘You are a great God, a powerful God, but actually I believe there are still some other gods beside You, even if they are not as powerful as You’. This is why we cannot believe that there are demons or a Devil in existence as well as the true God. This is just the mistake Israel made. Much of the Old Testament is spent showing how Israel displeased God by believing in other gods as well as in Him. We will see from the Bible that the “demons” people believe in today are just like those false gods Israel believed in.

Pasted from <http://www.realdevil.info/4-1.htm>