To get the full effect of the opening of Ulysses you need to know that Mulligan is mocking the Catholic mass. "Introibo ad altare dei" - "I shall go up to the Altar of God" aren't just any latin words from the bible, they are the fist words uttered by the priest. To which the altar boy would have answered (the latin phrase meaning) "The God who gives joy to my youth." This isn't common knowledge anymore but would have been familiar to many if not most of Joyce's readers. (As it was to me, since I had been an altar boy back when dinosaurs walked the earth and the mass was still said in Latin.) Mulligan isn't just mocking the mass with his ersatz rituals, he's also mocking Stephen, whom he considers too serious. And finally, Joyce is implicitly tell his readers that reading this book is going to be a sacred experience, like going to mass. No doubt many readers were scandalized and put the book down immediately.