This is the start of a beautiful friendship. Sebastian’s eccentricities captivate Charles’s attention and draw him into the "enchanted garden" he so hoped to find at Oxford.
Anthony Blanche’s long lecture to Charles over dinner sets us up for all the novel’s greatest conflicts. He calls Julia a "heathen," points out that Sebastian is essentially just an insipid bore, warns Charles of the entire Flyte family but particularly Lady Marchmain, and draws our attention to what he considers the greatest threat to Charles’s artistry: charm.
Sebastian’s attempt to solve the conflict (his family, his religion) by drinking only makes things worse. He grows more and more depressed as he sinks deeper into self-imposed isolation. On top of that, he and Charles both have to deal with Samgrass, a.k.a. The Most Annoying Family Friend Ever. And that’s all before Charles falls in love with Julia – despite each of their marriages to another.
After several months of anticipatory death-bed action, Lord Marchmain finally returns to Catholicism, moments before he dies. This is the event that spurs Charles’s own later conversion.
Charles’s foreshadowing metaphor of the ice fisher minutes away from a devastating avalanche is a good clue that something’s up with Julia and his relationship. It’s only a matter of time before the situation comes to a head. We’re also wondering who is going to end up living at Brideshead, since Lord Marchmain has promised it to Charles and Julia, but we know but we know from the prologue that this isn’t the end result.
You can definitely feel the novel winding down even as Julia ends her affair with Charles. (And not just because you notice you’re twenty pages from the end, either.) When Charles hears the latest news from Nanny Hawkins, it’s classic denouement territory, as information is revealed and any lingering questions answered.
Amazingly, Charles has found faith and become a Catholic in between the end of his narrative and the start of the epilogue. The novel’s conclusion is surprisingly optimistic, and you can read all about it in "What’s Up With the Ending?"