All that exists in this universe is either positive or negative. Wherever there is negation here, then there must be an affirmation there. In electrical circuits there are negative and positive polarities. Negative and positive must be joined to complete the electrical circuit & for a bulb to give light. If either of these is not there, there will be no flow of electricity and the bulb will not give light. Both the positive & the negative are necessary to complete the circuit. 

Everything that exists in this world exists because of the combined contribution of the negative and the positive - just as the two wheels of a bullock cart or the two legs of man have to make a combined contribution. In this same way, whatever exists has to have two poles, one negative and the other positive. 

But if God is the negative, then from where will the positive pole come? -> Another 'God' will have to be conceived. Satan is "God number two," the bad one, the one that will be blamed for everything that is deemed 'bad', the one that is the other polarity. The Devil has to be there, you cannot deny this polarity. Man has realized that existence is dual: if God is good, then the bad must also be allowed its existence. India is the only country that has not created any opposite entity to God. Christians, Mohammedans, Jews and Zoroastrians have all created a Devil. 

Having to create a Satan means there is little acceptance of God because then there is a constant, never-ending conflict.