“And he said to me, ‘Depart, for I will send you forth far hence to the Gentiles.' ”
He has been trying to impress on them that as a thorough Jew, he had only acted at the command of the God of the Jews all the way through. It had not been his choice. But when he told them what it was that God had next told him to do, his words were like petrol poured on a bonfire, turning a flame into a furnace. He informed them that God had then told him, ‘Depart, for I will send you forth far hence to the Gentiles.' Now strictly the idea of going to the Gentiles should not have upset them. The Old Testament had already spoken of the light being taken out to the Gentiles by the Jews, and especially by the coming Servant (Isaiah 42:6; Isaiah 49:6). And all Judaism looked for Gentile converts who would become proselytes, (although few actually sought them). And they actually welcomed into the synagogues questioning God-fearers (although not of course as equals). Furthermore he was pointing out that he had gone to the Jews first, as was always his mission, and it was only when they had turned him away that he had gone to the Gentiles. Thus he could claim to be fulfilling Isaiah's prophecy.
But in the context of his stated rejection by Jerusalem he was to their minds saying that he was going to the Gentiles instead of to Jews, because Jerusalem had rejected him and he no longer had any time for them, and that he was going to the Gentiles as Gentiles, not as those who had sought the Jewish fold. And in the light of the rumours about him this was too much for them. It appeared to confirm their worst fears. They had simply not taken in his argument, or possibly rather had not wanted to.
To Paul it was, of course, all perfectly logical. He probably could not see how they failed to understand it. And it all appeared to him so reasonable. He was a true Jew and had been called by the God of the Jews in a revelation in which had been revealed to him the Shekinah glory. How could he not then, as a true Jew, obey Him? But the problem was that it both threw the blame on them, which they did not like, and that it involved doing what horrified their ‘righteous' souls, going to the Gentiles direct. That might be all right for the Messiah or The Prophet when He came, but not for people like Paul.