Manzoni's novel is a massive panorama of 17th-century northern Italian society. The Catholic religion played a central role in that society, so it is fitting that it does so in the novel, as well. The book's title refers to the engaged couple Renzo and Lucia, poor but virtuous young people from a Lombard village who find themselves unable to get married when their local priest, Don Abbondio, refuses to perform the ceremony. A rapacious nobleman, Don Rodrigo, has forbidden Lucia to marry Renzo because he wants her for himself. Don Abbondio's cowardice is emblematic of the failings of the Church—the all-too-human personal failings of priests in any place and time, and the unfortunate alliance between the clergy and the secular power structure.
Weakness and immorality in the religious life are also manifest in the character of "the nun of Monza," to whom Lucia is entrusted while fleeing Don Rodrigo. The nun, Gertrude (based on a real historical figure), joined the convent for the wrong reason—family pressure—and then fell in love with a criminal, referred to in the novel as "the Unnamed." Under his influence, she conspired in the murder of another nun.
But these unsavory depictions should not give the reader the idea that Manzoni is anti-Catholic. He is a realist about the Church, accepting of the fact that it is made of up human beings, but ultimately, in The Betrothed, the Church is the source of salvation and the force that drives the plot to its happy conclusion.
Fra Cristoforo, a Capuchin brother, champions Renzo and Lucia's cause when Don Abbondio is too weak to do so. By challenging Don Rodrigo directly, he shows himself to be a worthy exemplar of Catholic virtue. More important, though, is Cardinal Federigo Borromeo, the archbishop of Milan. Although he is a high-ranking figure, entwined with the power structure, he incarnates all the qualities that a prince of the Church should possess. During the 1630 plague in Milan, which is a major episode in the novel, Cardinal Borromeo ministers to the needs of the sick in a truly saintly manner. And finally, through his influence, the Unnamed repents for his sins. Having held Lucia captive at the behest of Don Rodrigo, the Unnamed releases her and brings her back to her home town so that she can marry her beloved Renzo. Justice is served at last, and Manzoni makes it clear that he sees the Catholic Church as the only true source of justice on earth, even though some of its servants are fallible.