The overarching theme in The Thorn Birds is the conflict between love and ambition.
In the story, at least four characters sacrifice love on the altar of ambition. Father de Bricassart rejects Meggie and his love for her in order to realize his ambitions in the Church. Meanwhile, Luke O'Neill rejects an emotional connection with Meggie and his daughter in order to fulfill his ambitions in the sugarcane business.
Dane, Meggie's son, also chooses the Church; he disappoints the mother he adores in order to follow in the footsteps of his idol, Father de Bricassart. Ironically, he never discovers that the priest is his biological father. Meanwhile, Fiona (Meggie's mother) is rejected by her half-Maori lover, who chooses to fulfill his political ambitions at the expense of his love for Fiona.
The personal choices of these characters demonstrate the destructive nature of ambition and how it results in tragedy. Both Fiona and Meggie are jilted by the men they love. Their revenge, however, is bittersweet. Fiona hides her son, Frank's true paternal heritage, for as long as she can, but she eventually loses him. In the book, she warns Meggie that her own deception will meet the same, bitter end: Meggie will lose Dane, just as Fiona lost Frank.
Fiona's prophecy comes true: during his vacation in Greece, Dane has a heart attack after saving two German women caught in an undertow. He dies, and Meggie is forced to reveal the truth about Dane's paternal heritage to Father de Bricassart.
As for Luke O'Neill, he is never able to repair his relationship with Meggie. The couple remains estranged, and Justine (the couple's daughter) never develops a relationship with her father. So, the main theme of the novel is the conflict between love and ambition and the resultant tragedy from such a choice.