How did the gospel shift from being Jew only to being mostly Gentiles?

Les teaches that by Acts 28 Paul had finished his missionary journeys and was imprisoned in Rome. Paul called the Jewish leaders in Rome to meet him and reviewed how all the Old Testament covenant promises had been presented to Israel, but they rejected them when they crucified their Messiah. They left in discord and disagreement and could not believe. Then Acts 28 says, 'Be it known therefore unto you that the salvation of God is sent to the Gentiles.' Les points out this does not mean all Gentiles would be saved, but they would hear it, and Scripture shows they have; that is why the world stands guilty and Romans 1 says they are without excuse. With the advent of the Apostle Paul going out, it became predominantly Gentiles—'Gentiles only with exceptions,' and the exceptions are Jews. Jews can become believers in this Gospel, but they are few and far between. By far most of the calling out into the body of Christ is from among the Gentiles. It was a complete flip-flop: once Jew only with some Gentile exceptions, now Gentiles with some exceptional Jews coming in. Paul then refers to Romans 11.