You are
completely wrong. God and Satan haven't been "fighting each other for eternity." Lucifer was practically God's right hand man until he decided that he was more fit to rule over Heaven than God was and he conviced...I don't remember, [large number] of other angels that he should be ruler as well and they fought God and his loyal army and got smited. He was an Archangel, if you knew the Bible at all you'd know that's like the top tier of Angels. He exists because God
allows him to, God is supposed to be "omnipitent and omnipresent."
That being said, I'm basing my knowledge mostly off of
Paradise Lost and I haven't read that in a while but... somebody's got to answe poor Zoidberg's questions correctly.