Book by Morris L. West, published in 1959.
It follows an English priest, Blaise Meredith, who is dying of cancer. He is sent to investigate a man proposed for sainthood in a desperately poor area of rural Italy. Meredith has been chosen to be Promoter of the Faith, or Devil's Advocate, in the canonization process.
The candidate for sainthood, Giacomo Nerone, had appeared in the Calabrian villages of Gemello Minore and Gemello Maggiore during World War II. Within a few months, he was executed by the partisans. His actions in that brief period, between arrival and death, plus the miracles attributed to him thereafter, are the subject of the investigation. Meredith finds the villagers of Gemello Minore still deeply divided over Nerone's life and death, more than fifteen years afterwards.
The theme of the book is redemption, not just of a mysterious figure in the darkest days of the War, but of many of the people Meredith meets in the investigation, and, finally, of Meredith himself.
A film version of the book was made in West Germany in 1978, entitled Des Teufels Advokat. It was scripted by West himself, directed by Guy Green, and starred John Mills as Meredith. Although it was a German production, it was an English-language movie.