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.