what is the religious theme in the novel?

Religion isn't presented as a major theme in the story but it always lurks in the background. Tolstoy wrote this book after his conversion to a more orthodox form of Christianity. It isn't until Ivan's last moments before dying that he considers God as the meaning of life and God's involvement in his sufferings and even penance. Still, the idea of God, punishment and mercy remains just under the surface of things, seldom directly dealt with.