But if you believe in God, and believe that the Bible is His inspired word, then you have to accept the reality of Satan. The Bible is very clear on this. Satan was once the archangel Lucifer. Lucifer means "Morning Star" and the Bible tells us he had beauty, wisdom, great abilities--he was perfection. But he had pride. Lucifer wanted to be worshiped as a god, actually, as Thee God. Lucifer led a rebellion with 1/3 of all the angels to overthrow God (so much for wisdom!) and of course, he lost. Lucifer and his angel followers were cast down from Heaven becoming Satan and demons.
God in his mercy, did not destroy Satan. He actually, in a way, gave Satan what he wanted. God gave Satan control of this world, the Bible calls him the "Prince of this World". And God gave people free will. We are free to worship God, or worship Satan, or ignore (not believe in) both.
Why does Satan hate us so much? Because God loves us so much. He will stop at nothing to separate us.
Satan is wise, he is a great deceiver. And we are so willingly deceived.
Satan has first succeeded in people not believing he is real. How does he do this? First, his name is used so flippantly, that it no longer holds power. We say things like, "the devil made me do it", or "what the devil is that?" or we play "devil's advocate". We dress up like him at Halloween. He has become is a caricature, a cartoon in our culture. He is mythological, so he is not real.
This is Satan's greatest triumph.
If we do not believe in him, then it is so much easier to separate us from God.