Father Sergius by Leo Tolstoy

Young Prince Stepan seems destined for great things. However, on the eve of his wedding, he discovers that his fiancée has had an affair with the Tsar. Devastated, Stepan enters a monastery, where he becomes known as Father Sergius. He spends years in isolation, but is unable to attain full faith. He is eventually seduced by a young woman, and then leaves the monastery, intent on making amends with those who he has wronged.