Why was the idea of God bringing Gentiles to reconciliation outside Judaism incomprehensible to the Jerusalem believers?
It was incomprehensible because the gospel Paul preached among the Gentiles was something totally different from what the Jews in Jerusalem knew about. It was an advance, yet they could not grasp that God would now go to the Gentile, pagan, idolatrous world and bring those people to reconciliation with Himself outside Judaism.
Les teaches that Paul was writing to churches in Asia Minor who were being deluged with Judaizers trying to bring Gentile believers under the law, legalism. Paul went up again to Jerusalem fourteen years after his conversion on the road to Damascus, with Barnabas and Titus. This trip was by revelation, a supernatural trip God instructed and led. He communicated to Peter, the eleven, and other leaders of the Jerusalem believers the gospel he preached among the Gentiles. Les explains that this was something totally different from what the Jews in Jerusalem knew about; it was an advance, but they could not comprehend that God was now going to the Gentile, pagan, idolatrous world and bring those people to a place of reconciliation with Himself outside of Judaism. That was incomprehensible to them.
And I went up by revelation, and communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.