The most important theme is that of forgiveness and love. Daniel is entirely ruled by his emotions and his need for revenge; he is driven by anger and not thinking about the larger consequences of his actions. For example, when he and Rosh's bandits attack Roman forces, the Romans take it out on the villagers because they cannot find Rosh and his people. Over the course of the novel, both through his friendship with Joel and Thacia and through the teachings of the preacher Jesus, Daniel realizes that he cares about more than just himself; he needs to love and accept other people, even Romans, or his anger will destroy him.
Was it possible that only love could bend the bow of bronze?He sat trembling, glimpsing a new way that he would never see clearly or understand.
Almost with the thought the terrible weight was gone. In its place a strength and sureness, and a peace he had never imagined, flowed around him and into his mind and heart.
(Speare, The Bronze Bow, Google Books)
This theme can be clearly seen as Daniel tries to reconcile his own feelings of anger with the clear leadership he feels when he meets Jesus. In Daniel's eyes, the preacher is the perfect leader for a revolution, and he cannot understand why Jesus is not leading the rebellion. It is only later that Daniel realizes Jesus means to change people through their hearts, not through the sword, and that this is ultimately a more effective method of rebellion.