One of the strongest Christian themes in The Thorn Birds appears to be that the wages of sin is death. Those who engage in sinful activities lose someone dear to them. Fiona had a child before she was married to Paddy Cleary; that child, Frank, eventually winds up in prison for thirty years. Meggie loses both de Bricassart and Dane. De Bricassart loses the son he never knew and eventually his life. God, as envisioned by Colleen McCullough, is not only a jealous God; he is a vengeful one, as well.

However, other sins seem less frequently punished. De Bricassart can do the Clearys out of their inheritance with impunity; he rises in the Church anyway. If he suffers pangs of conscience, those pangs do not deter him from his clerical ambitions. Dane is presented as perhaps the most pure-hearted character in the story, and for that and his heroism, he drowns, the sins of the parents visited upon him.

It remains for Justine, not presented as a likable character at all, to effect a degree of healing in the family. She suffers great guilt when Dane dies, though she was not responsible. Meggie finally rises to an unselfish act by freeing her from her need for repentance, allowing healing to come to the women in the family and allowing Justine finally to love and be loved.

The author's most famous book is her most purely entertaining, with few obvious moral lessons. It follows three generations of an Australian family from 1915 to 1969. The story highlights the oppression and limited freedom of women dependent on men, with few alternatives available to them. Although the women voice some early feminist ideas, only the woman from the third generation successfully creates an independent life. However, her fulfillment is only achieved after marriage, which marks the novel's ending.

The protagonist, Meggie, nurtures a deep, enduring affection for the ambitious priest, Ralph de Bricassart. Their love is fleetingly fulfilled, leading to the birth of an illegitimate son who eventually becomes a priest himself. Meggie also has a daughter with a man she marries mainly due to his resemblance to Ralph; however, their marriage is marked by discontent. The narrative is heavily influenced by the factors that separate Meggie and Ralph, especially the priest's steadfast dedication to his vocation and his immense ambition.

The novel delves into the challenges and sacrifices of a religious life through the character Ralph de Bricassart. It portrays his struggle with the impossible pursuit of spiritual perfection. Although Ralph's affection and longing for Meggie are clear, he acknowledges that his genuine vocation lies within the upper echelons of the Church, a position he ultimately achieves.