The friends choose a symbol from Psalm 18, “He trains my hands for war, so that my arms can bend a bow of bronze.” The Psalm refers to God’s ability to give strength to those who fight on his behalf. If left to human strength, bending a bronze bow would seem to be impossible. The point of the symbol is that God makes such things possible. For now, the friends think of feats of rebellion against Rome, but the symbol will change as the story develops.