In The Bronze Bow, why does Joel think Jesus needs to be warned?
After Joel's rescue, he returns to his home and finds out that the Rabbis at the Jewish Temples are angry about the preachings of Jesus. They believe that he is changing and altering the laws they received in the Old Testament, and that alteration is against the Word of God. This internal dissent means that Jesus has enemies on both sides, from his own people and from the Roman occupiers. Joel believes that the threat from within might actually be more dangerous than the threat from without.
"I mean the elders of the synagogue. The rabbis and the scribes. They can't understand him. They're furious at the things he says and does. He is too free with the Law. They say he is trying to destroy all the authority of the Temple. Some of them even say he is in league with the devil. [...] Some of them hate him so much -- I think they would kill him if they could."
(Speare, The Bronze Bow, Google Books)
The change between the original teachings and laws in the Old Testament and the new interpretations and sermons of Jesus is one of the most important aspects of the split between Judaism and Christianity. In earlier times, religious disputes were more often cause for violence; Joel is scared that the Rabbis might take the matter into their own hands to remove heresy. Therefore, while Jesus is fully aware of the threat from Rome, Joel believes that he should be cautious even at home.