The central theme seems to be the relationships between passionate sexual love, sterile and exploitative lust, and the all-encompassing love of God.
Sean the priest spends his life in love with Paul's wife, Nora, while Paul is incapable of loving her. Greeley believes that the love of a woman offers a powerful and fruitful model for humans of God's love. Sean Cronin is a fallible human being who is yet faithful to his commitment to the priesthood.
Nora is a sacrament, a powerful symbol of what God's love is like. Greeley seems to want to shake up his readers' preconceptions about faith and morality, and to open their hearts to the essence of Christianity, which is love.