Complex sentence: The central thesis of the book is that Jesus, while free from sin, was still subject to every form of temptation that humans face, including fear, doubt, depression, reluctance, and lust.
Split into simpler sentences: The central thesis of the book is that Jesus was a mortal man. While free from sin, he was still subject to every form of temptation that humans face, including fear, doubt, depression, reluctance, and lust.