Most of Waugh's novels before Brideshead were satires: Scoop was a satire about the journalism industry, for example; Vile Bodies was a satire about extravagant young people in London society between the two World Wars.
Brideshead has a very different tone and very different themes from Waugh's other novels. In general, Brideshead is considered the most "Catholic" of Waugh's novels, for many reasons. One of the main reasons is that the book deals heavily with difficult themes such as the ones your question raises: sin and salvation, or the fall and redemption.
Many of Brideshead's characters fall: Lady Marchmain is abandoned by her husband, who leaves her for another woman. Sebastian Flyte, Charles Ryder's friend from Oxford, eventually leaves England for Tunisia, where he descends into alcoholism. Charles himself marries a woman, falls out of love with her, and eventually divorces her. Julia, Sebastian's sister, also divorces her husband.
The effect of these "falls" is to destroy the idealism of Charles' first summer at the Brideshead estate, when the family seemed wealthy and carefree and above the difficulties of life. Charles doesn't return to Brideshead until after the Second World War, when he sees that the church inside is being used for worship by the soldiers. This signifies that the estate, and the unhappy people who inhabited it, did not ruin God's plan, but only delayed it.