Before hearing Daniel's story, Thacia is confused as to why he associates with Rosh. In her mind, Rosh is a bandit, doing far more harm than good. She also cannot see Rosh as an emissary of God, believing that violence is not the answer to the Roman occupation. Daniel tells the story of how his father and uncle were crucified by the Romans, how his mother died of exposure, and how his sister Leah was traumatized. After that point, although Thacia still believes in her peaceful mission, she is far more understanding of Daniel's drive, and even joins them in their oath.
"Yes," she said steadily. "I do see. If I were a boy I would make a vow too."
Suddenly Joel's fire leaped up in her face. "Why can't I?" she cried. "Why can't a girl serve Israel too? What about Deborah and Queen Esther? Let me swear it too, Joel! I promise to help you."
(Speare, The Bronze Bow, Google Books)
Thacia is also the person who comes up with the oath itself -- "For God's Victory" -- and the symbol of the bronze bow for their rebellion. Although it is not obvious at the time, this is Thacia's way of influencing the eventual direction of the rebellion. Since "only love can bend the bronze bow," she knows that with time and experience, Daniel will come to the realization that hatred is not the path to success. Instead, he must learn to forgive and love, and through that love he will affect far more people than through hatred.