Damien Ross is an Exorcist, an exorcist who no longer believes in the teachings of the church. In fact, he no longer believes in God. He remains a priest, though, because his life is easy.
Zell is a demon, one who possesses a body once every six months and demands the Damien be the exorcist he deals with.
When Damien tells Zell that the exorcism he is performing will be his last, Zell offers him a deal. If Damien goes public with his condemnation of all organized religions, Zell, by possessing people, will enable Damien to perform miracles and, therefore, convince the world that he is the second coming of Christ. Zell has an ulterior motive, though. He is not really a demon, but a spirit trapped in this universe through some cataclysmic event. He believes that, with Damien's help, he can get back to his own universe. To achieve his goal, Zell must make Damien so popular that no one will deny him the resources he needs to send Zell back.
Damien agrees to the plan. When a crazed zealot shoots him, though, and he seemingly dies. Three days later, he returns to life, thereby cementing the world's belief that he is, indeed, the second coming.
Zell's plan is apparently successful. Without Zell's help, Damien knows that he cannot perform any more miracles. He is wrong, though, and begins to belief that he is somehow divine.