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 popular book is her most purely entertaining, with few overtly moral messages, as it traces three generations of an Australian family from 1915 to 1969. It does, however, show the subjugation and lack of freedom for the women who are dependent on men and have few options without them. They register some pre-feminist protests, but only the woman of the third generation breaks away for a life of her own, and it is not satisfactory either until she marries, which is the last event in the novel.

The central character, Meggie, carries a lifelong love for an ambitious priest, Ralph de Bricassart. It is briefly consummated, resulting in an illegitimate son (who later becomes a priest); she also has a daughter by a man she marries (unhappily) mainly because he resembles Ralph. The forces that keep Meggie and Ralph apart dominate the story, including the priest's inevitable placing of his job (and his incredible and successful ambition) ahead of his love.

In the character of the priest, Ralph de Bricassart, the novel also touches on the demands of the religious life, the sacrifices required, the impossibility of achieving spiritual perfection. We see Ralph loving and desiring Meggie, but knowing that he is best suited to the high offices in the Church that he eventually holds.