The quote is significant because it symbolizes the union of Joel and Thacia with Daniel, joined together with the common purpose of working to free the Jewish people from Roman subjugation. Daniel, whose father was crucified by the Romans, has vowed to do whatever he can to rid the land of the oppressors, and has "longed to share the burden that he (has) carried alone for so many years". Joel and Thacia are galvanized into action after hearing Daniel's story, and join him in resolving to "do anything...to make the country free again". The three young people swear on the Book of Enoch to "fight for Israel...for God's Victory", and the bronze bow acclaimed in the Book of Enoch will be the sign by which they will communicate.
The quote about the bronze bow,
"He trains my hands for war, so that my arms can bend a bow of bronze",
has a direct meaning for Daniel, Joel, and Thacia as well. Bronze is a metal known for its hardness; Daniel notes that, taken literally, "the strongest man could not bend a bow of bronze". It is Thacia who grasps the deeper meaning of the quote. She explains that "when God strengthens us we can do something that seems impossible" - like bend a bow that is known to be made of a material that man cannot bend, or free the land of Israel from under mighty Roman rule (Chapter 7).
I think it is saying that only Jesus can be the one to be right there beside us to bend the bow of bronze. Because it is made out of bronze and is "unbedable" to us, He is the only one who can bend it with his love.