Complex sentence: Peretti reportedly felt badly about his early books' success--he insisted that he had not yet developed his craft as a writer sufficiently, and began working with secular publishers and editors in an attempt to improve his writing.
Split into simpler sentences: Peretti reportedly felt bad about his early books' success and insisted that he had not yet developed his craft as a writer sufficiently. He began working with secular publishers and editors in an attempt to improve his writing.