Looks like it was inspired by the story Blue World
Robert McCammon wrote a short story called Blue World about 25 years ago or so detailing the story of a young priest (also named John) that was in his confessional when a young porn actress/prostitute confesses to him the sadness of her friends death.
The priest becomes obsessed finding who the young girl is and tracks her down and befriends her all the while trying to protect her from a serial killer
He even goes so far as buying magazines and x rated videos at an adult book store to put a face to her voice which he thought was unique
It is a really good story. Wish this movie had followed the story's plot line instead of this one
"I like simple pleasures like butter in my ass and lollipops in my mouth."