In the Bible, God tells us that he alone is God, that there is no other. None before him or after him, that he knows not one. The Bible tells exactly why people would worship other 'gods' in Romans 1-2. It tells how people turned from God, their creator, and began worshipping the creation, making idols of wood, gold and silver, etc. They worshipped the sun, moon, stars, earth, trees, rivers, animals, etc., the creation instead of the Creator. Paul goes so far as to state that the idols represent demons. This is the source of power behind these false gods. Satan's main goal is to have us worship him instead of God, which is why he fell in the first place, he tried to exalt himself above God. He promised Jesus all the power over all the kingdoms of the world if he would but worship him, so the power behind these false gods is real, it is from Satan and his demon followers. Same with witchcraft, the power behind it is real, but it is not from the Lord God in Heaven, whatever folks may say.