Catholicism is a significant theme of the book. Evelyn Waugh was a convert to Catholicism and Brideshead depicts the Roman Catholic faith in a secular literary form. Waugh wrote to his literary agent A. D. Peters, that "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 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 quotes 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.
The same themes were criticised by Waugh's contemporaries. Novelist Henry Green, 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."
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 question of whether the relationship between Charles and Sebastian is homosexual or platonic has been debated, particularly in an extended exchange between David Bittner and John Osborne in the Evelyn Waugh Newsletter and Studies from 1987 to 1991. In 1994 Paul Buccio argued that the relationship was in the Victorian tradition of "intimate male friendships", which includes "Pip and Herbert Pocket [from Charles Dickens' Great Expectations], ... Sherlock Holmes and Doctor Watson, Ratty and Mole (The Wind in the Willows)". David Higdon argued that "[I]t is impossible to regard Sebastian as other than gay; [and] Charles is so homoerotic he must at least be cheerful"; and that the attempt of some critics to downplay the homoerotic dimension of Brideshead is part of "a much larger and more important sexual war being fought as entrenched heterosexuality strives to maintain its hegemony over important twentieth century works". In 2008 Christopher Hitchens derided "the ridiculous word 'platonic' that for some peculiar reason still crops up in discussion of the story".
Those who interpret the relationship as overtly homosexual note that the novel states that Charles had been "in search of love in those days" when he first met Sebastian, and quote his finding "that low door in the wall [...] which opened on an enclosed and enchanted garden" (an allusion to Alice's Adventures in Wonderland by Lewis Carroll). The phrase "our naughtiness [was] high on the catalogue of grave sins" is also seen as a suggestion that their relationship is homosexual, because this is a mortal sin in Roman Catholic dogma. Attention has also be drawn to the fact that Charles impatiently awaits Sebastian's letters, and the suggestion in the novel that one of the reasons Charles is later in love with Julia is her physical similarity to Sebastian.
Waugh wrote in 1947 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 novel, Cara, Lord Marchmain's mistress, says to Charles that his romantic relationship with Sebastian forms part of a process of emotional development typical of "the English and the Germans". This passage is quoted at the beginning of Paul M. Buccio's essay on the Victorian and Edwardian tradition of romantic male friendships.