Thomas Aquinas is a priest sent by the Vatican to protect a girl prophesied to be the mother of Satan's child.
The morning after Satan inhabits the body of an investment banker (Gabriel Byrne), Thomas Aquinas is on a rooftop shooting at him in an attempt to kill him.
In late 1999, Satan possesses an investment banker (Gabriel Byrne) in a restaurant; he then destroys the restaurant, killing many inside. Suicidal and alcoholic former police detective Jericho Cane (Arnold Schwarzenegger), depressed since his wife and daughter (Denice D. Lewis and Renee Olstead respectively) were killed in contract killings, works for a private security company and blames God for his plight. Jericho and co-worker Bobby Chicago (Kevin Pollak) are assigned to protect the possessed banker. A priest, Thomas Aquinas (Derrick O'Connor), unsuccessfully tries to kill the banker.
What evidence does the movie provide informing Thomas Aquinas that Gabriel Byrne's character is in fact Satan?