Sin and temptation play central roles in this novel. Our protagonist goes through a period of indulging fully in his bodily lusts, which then leads to a swing in the opposite direction, an attempt at total piety. Joyce highlights the harshly binary nature (people either give in to all sins or no sins at all) of the Catholic-dominated Irish culture. In the end, the hero comes to the necessary conclusion that sin is a fundamental and unavoidable part of human nature, rather than something that can simply be eliminated through religious practice. One suspects that Joyce hoped that the reading public of the time would come to the same conclusion.
By the end of the novel, Stephen reaches the conclusion that sin is an inevitable and even beautiful part of human life.
Even at the end of the novel, after his "conversion" from Catholicism, Stephen remains obsessed by the categories of sin and guilt.