Daniel's original vow, at the age of eight, was to devote his life to the destruction of the Romans occupying Israel. Through his apprenticeship with Rosh, he has worked towards that goal, although he does not yet know that Rosh is only using Daniel's zeal to enrich himself. Later, when he renews his oath with Joel and Thacia, the language of the oath changes:
Thacia laid her hand firmly over her brother's. "For God's Victory," she repeated. They looked at Daniel, waiting. The three of us, Joel had said, taking him, who had always stood outside, into the close circle of their lives.
(Speare, The Bronze Bow, Google Books)
Daniel does not make the distinction until much later; while Joel is still young and driven by much of the same patriotic fervor that consumes Daniel, Thacia is smarter and knows that the men will destroy themselves long before they make any difference in the occupation. Instead, the new oath is to win "God's victory" which is not necessarily the same thing as killing Romans. Thacia knows that they need to change the minds of people from within, not kill those with whom they disagree. This coincides with the teachings of Jesus, who, after hearing Daniel repeat his oath, calls it representative of love instead of hate, even if Daniel does not yet realize it.
In the book, Daniel,Mathace, and Joel make an oath to get back their revenge (or kill) the Romans