The demon is a liar. He will lie to confuse us; but he will also mix lies with the truth to attack us. His attack is psychological, Damien. And powerful.
As far as God goes, I _am_ a nonbeliever. Still am. But when it comes to a devil—well, that's something else.
Perhaps evil is the crucible of goodness… and perhaps even Satan — Satan, in spite of himself — somehow serves to work out the will of God.