Father Brown (The Detective in the United States) is a 1954 mystery comedy film. It is based on the G. K. Chesterton Father Brown stories and was directed by Robert Hamer and stars Alec Guinness in the title role. Other featured actors include Joan Greenwood, Peter Finch, Cecil Parker and Bernard Lee. The screen adaptation was written by Thelma Moss, and is based on several of the Father Brown stories, notably “The Blue Cross”.Father Brown tries to transport an historically important cross to Rome engaging in battles of wits and faith with a thief and pursuing policeman. The thief, named Flambeau, is a master of disguise and is elusive, as Father Brown pursues him and tries to convince him to abandon his criminal career.
(The above is taken from Wikipedia) An obituary of Guinness notes:
His conversion to Roman Catholicism followed an episode during the 1954 shooting of Father Brown (called The Detective in the US), in which he played GK Chesterton’s cheery cleric. Walking back in the dark, still in a cassock, to the station hotel of a village near Macon after a drink in the local bar with Peter Finch, his hand was seized by a small boy, a complete stranger, who called him “Mon père” and trotted along beside him chatting in French. Despite his phony credentials as a cleric, Guinness felt strongly that the reality of this trust was important. When his 11-year-old son Matthew was temporarily crippled from the waist down with polio, Guinness had taken to dropping in on church and praying. Shortly after Father Brown, he joined the church of Rome.