City of God by Michael Jahn

In ten words or less: Mad killer haunts a NYC cathedral.

Review: The world's largest Gothic cathedral is the Cathedral of St. John the Divine in New York City. A killer has nailed a threat to the door. "The pleasure seekers of the City of Man will be punished for encroaching on the City of God. I am the protector of his temple, and I speak for him."
The first victim, a film festival organizer, was bludgeoned to death. While Lt. Bill Donovan searches among the mazes and cellars of the cathedral, Marcie's friend Jennifer is stabbed to death while running the NYC marathon.

Why bother? There are plenty of suspects and plenty of hiding places. Donovan must catch the killer before he becomes the next victim. For those who enjoy big city police procedurals.