The theme of much of Marshall's works is religion, with a focus on Roman Catholicism. His first great success, "Father Malachy's Miracle", is about an innocent Scottish priest whose encounter with sinful behaviour causes him to become involved in a miracle. A number of his later novels also deal with clergy who are faced with temptation but manage to triumph in a modest and humble manner (e.g., "The World, the Flesh, and Father Smith" (AKA "All Glorious Within") (1944), "A Thread of Scarlet" (AKA "Satan and Cardinal Campbell") (1959), "Father Hilary's Holiday" (1965), "The Month of the Falling Leaves" (1963)). Other books centered on religious issues deal more with Catholic doctrine and its relationship to modern life than with personal responsibility, such as "The Bishop" (1970), "Peter the Second" (1976), "Urban the Ninth" (1973) and "Marx the First" (1975).