I always understood what was in the case to be Marceluses soul that he had sold to the devil.
He hired his "hitmen" Vincent and Jules to get it back for him. Though Vincent and Jules were fallen angels. So when "god interveined" it sent Jules on the path to righteousness and delievered him from evil. The fact that Vincent still balked at that, made him still a sinner, hence he was killed in the end and not spaired like Jules was.
But that is my take on it.
Thats deep .
Cool perspective I love it .