Time is running out and the Devil needs to take over a man's (Gabriel Bryne) body so it can impregnate Robin Tunney, thereby producing the antichrist and opening up the gate from hell to earth. She's the right choice since her "stars" line up so well.
This has to be done before the end of 1999.
All is going well until Arnold runs over Gabriel Byrne with a subway train and then blows him up.
With no one else to turn to, and time running out, the devil takes over Arnold's body.
Arnold is too powerful and smart for the devil and kills himself by falling on a statue's sword, thereby thwarting the devils plans.
The Devil in a giant fireball, goes back to hell, having to wait until 2999 to try again.
(999 is 666 upside down, get it?)