Complex sentence: Despite his beliefs, he had an adulterous affair with Anna Noyes during his marriage to Meta, writing an unpublished novel about the affair called "Love's Progress", a sequel to "Love's Pilgrimage".
Split into simpler sentences: Despite his beliefs, he had an adulterous affair with Anna Noyes during his marriage to Meta. He wrote a novel about the affair called "Love's Progress", a sequel to "Love's Pilgrimage".