Daniel makes three vows during the book: one to himself, one with Joel and Thacia to God, and a repetition of the second vow to Jesus. His first is made at the age of eight, when he witnesses his father and uncle crucified: Daniel swears to live his live in pursuit of vengeance, to his dying breath.
"I vowed I would pay them back with my whole life. That I would hate them and fight them and kill them. That's all I live for."
His second vow comes later, as his mind begins to expand; he influences Joel and Thacia to join in his mission, but they move the vow in a different way:
"Then we will make a new vow," [Joel] said. "The three of us together. We'll swear to fight for Israel--for--for--" He hesitated.
"For God's Victory," said Thacia swiftly.
(Speare, The Bronze Bow, Google Books)
The second vow changes Daniel's focus. He is no longer driven by his need for direct revenge, but instead is working towards a freed Israeli nation. Because of this, his mindset towards Rosh changes; Daniel starts to see that Rosh never cared about avenging the dead, but only about enriching himself. Because of this new vow, Daniel's heart and mind are opened to the possibility of leaving his anger behind and embracing love and forgiveness; this is shown directly in his third vow, to live and die for God's victory, which he only understands when he accepts the teachings of Jesus.