Catholicism becomes a significant theme of the book. Evelyn Waugh was a convert to Catholicism and the book is an attempt to express the Roman Catholic faith in secular literary form. Waugh wrote to his literary agent A. D. Peters, "I hope the last conversation with Cordelia gives the theological clue. The whole thing is steeped in theology, but I begin to agree that the theologians won't recognise it."
The book brings the reader, through the narration of the initially agnostic Charles Ryder, in contact with the severely flawed but deeply Catholic Flyte family. The Catholic themes of divine grace and reconciliation are pervasive in the book. Most of the major characters undergo a conversion in some way or another. Lord Marchmain, a convert from Anglicanism to Catholicism, who lived as an adulterer, is reconciled with the Church on his deathbed. Julia, who entered a marriage with Rex Mottram that is invalid in the eyes of the Catholic Church, and is involved in an extramarital affair with Charles. Julia realizes that marrying Charles will separate her forever from her faith and decides to leave him, in spite of her great attachment to him. Sebastian, the charming and flamboyant alcoholic, ends up in service to a monastery while struggling against his alcoholism.
Most significant is Charles's apparent conversion, which is expressed very subtly at the end of the book, set more than 20 years after his first meeting Sebastian. Charles kneels down in front of the tabernacle of the Brideshead chapel and says a prayer, "an ancient, newly learned form of words" – implying recent instruction in the catechism. Waugh speaks of his belief in grace in a letter to Lady Mary Lygon: "I believe that everyone in his (or her) life has the moment when he is open to Divine Grace. It's there, of course, for the asking all the time, but human lives are so planned that usually there's a particular time – sometimes, like Hubert, on his deathbed – when all resistance is down and grace can come flooding in."
Waugh uses a quotation from a short story by G. K. Chesterton to illustrate the nature of grace. Cordelia, in conversation with Charles Ryder, quotes a passage from the Father Brown detective story "The Queer Feet": "I caught him, with an unseen hook and an invisible line which is long enough to let him wander to the ends of the world, and still to bring him back with a twitch upon the thread." This quotation provides the foundation for Waugh's Roman Catholic treatment of the interplay of free will and grace in the moment of conversion. Aside from grace and reconciliation, other Catholic themes in the book are the Communion of Saints, prayer, faith and vocation.
The same themes were criticised by Waugh's contemporaries. Henry Green, a fellow novelist, wrote to Waugh, "The end was not for me. As you can imagine my heart was in my mouth all through the deathbed scene, hoping against hope that the old man would not give way, that is, take the course he eventually did." And Edmund Wilson, who had praised Waugh as the hope of the English novel, wrote "The last scenes are extravagantly absurd, with an absurdity that would be worthy of Waugh at his best if it were not – painful to say – meant quite seriously." A reviewer of the book at the time of its publication regarded it as an apologia for Catholicism.
Nostalgia for an age of English nobility
The Flyte family is widely found to symbolise the English nobility. One reads in the book that Brideshead has "the atmosphere of a better age", and (referring to the deaths of Lady Marchmain's brothers in the Great War) "these men must die to make a world for Hooper ... so that things might be safe for the travelling salesman, with his polygonal pince-nez, his fat, wet handshake, his grinning dentures".
According to Martin Amis, the book "squarely identifies egalitarianism as its foe and proceeds to rubbish it accordingly".
Charles and Sebastian's relationship
The precise nature of Charles and Sebastian's relationship remains a topic of debate; whether they are simply close friends or if Waugh hints at a sexual relationship between the two is not definitely established. Given that much of the first half of the novel focuses on the initial encounter, blossoming friendship and eventual estrangement of these central characters, this issue continues to pique the curiosity of readers.
Readers who interpret the relationship as overtly homosexual quote such lines as the fact that Charles had been "in search of love in those days" when he first met Sebastian, and his finding "that low door in the wall ... which opened on an enclosed and enchanted garden" – an image that some interpret as a Freudian metaphor for homosexual sex, though it recurs when Charles is expelled from Brideshead by Lady Marchmain, suggesting it refers more generally to the glamorous world Sebastian represents: "a door had shut, the low door in the wall I had sought and found in Oxford." (A reference to Alice's Adventures in Wonderland by Lewis Carroll).
The line "our naughtiness [was] high on the catalogue of grave sins" may also be a suggestion that their relationship could have a homosexual element, which, if acted upon, would be a mortal sin in Roman Catholic dogma. Reference is made at one point to Charles impatiently awaiting Sebastian's letters. It is also suggested in the book that one of the reasons Charles is later in love with Julia is the similarity between her and Sebastian.
Another interpretation is that Charles and Sebastian had a passionate yet platonic relationship, an immature albeit strongly felt attachment that prefigures future heterosexual relationships. Waugh himself said that "Charles's romantic affection for Sebastian is part due to the glitter of the new world Sebastian represents, part to the protective feeling of a strong towards a weak character, and part a foreshadowing of the love for Julia which is to be the consuming passion of his mature years." In the book, Cara, Lord Marchmain's mistress, says to Charles that his relationship with Sebastian forms part of a process of emotional development typical to "the English and the Germans".