Sorry for not noticing this sooner.
OP writes, “A Jewish friend…asked me what I would do if my brother, hypothetically, said that God was speaking through him or to him. I answered that I would tell my priest or bishop and that I would believe whatever Rome told me (so if the authorities in the Church told me he was lying, I would believe that).”
–My answer would be completely different. I’d rely on what I saw my brother do. Specifically,could he heal the blind? Calm a storm at sea? Feed 5 thousand, miraculously?
Clearly, Jesus did some things which appeared miraculous to those who saw them. We can debate if he really fed 5 thousand or 4 thousand, or if those stories are 1 or 2 miracles…but that is a detail. Clearly he did “something,” multiple times. such that some people literally gave up their entire lives to follow him.
Now, as to why rejecting Jesus might be wrong, I am reminded of the old addage, “who do you believe…me, or your lying eyes?” A person could have seen, and believed, but allowed themselves to disbelieve, for a host of (bad) reasons. Perhaps a Jew wanted to believe, but feared becoming an outcast…or was afraid of how belief might change their lives for the worse. Perhaps they were just afraid of change. Perhaps the Messiah was not who a Jew expected, and they feared hw their beliefs, worldview, etc. would change if they allowed themselves to believe.In any case, such rejection would be wrongful.
The wrong behavior of the Rabbis would be worse, inasmuch as they could have more easily seen, for example, how Jesus fulfilled the prophecies of the OT but also knew that their own stations would be threatened if this Jesus really was “the One” they’d been waiting for. These folks were the ones asking, “do you believe me, or your lying eyes?” Some Jews followed the advice of their rabbis, sure; but some of them (the laity) should have known better than to follow that advice; worse, some of them – the rabbis, and the ones who knew what the Messiah would look like, probably did not believe because they did not want to believe.
The book, “The case for Christ,” although written by a Protestant, quotes a Jewish convert to Christianity who converted after learning how closely Jesus matched up to the Jewish Scriptures’ prophetic writing about what the Messiah would look like (not physically, of course; rather, how he would act, his characteristics, etc.