I suppose it would be a flaw in Waugh's novel if you had to be Catholic to "get" it. But I really don't think that's case. I do think you have to avoid being obtuse. I don't know how old Troy Patterson is, but consider this passage that he quotes:
The languor of Youth—how unique and quintessential it is! How quickly, how irrevocably, lost! .... [L]anguor—the relaxation of yet unwearied sinews, the mind sequestered and self-regarding, the sun standing still in the heavens and the earth throbbing to our own pulse—that belongs to Youth alone and dies with it. Perhaps in the mansions of Limbo the heroes enjoy some such compensation for their loss of the Beatific Vision; perhaps the Beatific Vision itself has some remote kinship with this lowly experience; I, at any rate, believed myself very near heaven, during those languid days at Brideshead.
The sad fact is that, around age 39, that is how one actually tends to feel about one's lost youth, especially if it was a rather footloose and carefree one. The melancholy sentimentality, the self-pity are all authentic. Of course, life goes on, one moves on, and one is embarrassed to have felt that way, however briefly. But that is how one did feel, and Waugh has captured it perfectly in all its self-indulgent grandiloquence. I might add that I read the book when I was in college and found it unsettling precisely because I felt sure I would face just such a moment sometime in my own future. As indeed I did.
I have just ordered a copy of the book. I am so tired of reading stuff about it that I think misses its point that I have decided to re-read it after all these years and see what I think and feel now.