Because the point of being an athiest is to not believe in god or the devil and if you are a christian it is required to believe in god and the devil. So either you believe or you don't you can't be both it's imposible. I an athiest so I don't believe
like tracy said it is like cutting the heat on and then leaving the window down it is pointless you cant love or be on both sides that would be luke warm These are the words of the Amen, the faithful and true witness, the ruler of God’s creation. 15 I know your deeds, that you are neither cold nor hot. I wish you were either one or the other! 16 So, because you are lukewarm—neither hot nor cold—I am about to spit you out of my mouth. king james version
To expand the context a little might help. When you say Christian, do you mean a person who believes Jesus is the Christ, or a person of virtue living a life of love, forgiveness, and empathy. It is my view that it can be proven that so called Christian virtues are simply co-opted values that have made humans successful for many thousands if not millions of years. So go ahead and live a virtuous life of empathy because that's what it is to be human.