What is the flip-flop in who is called into the body of Christ?
The flip-flop is that the calling into the body of Christ shifted from being Jew only with a few Gentile exceptions to predominantly Gentiles with only exceptional Jews coming in. Israel rejected the covenant promises and crucified their Messiah, so salvation was sent to the Gentiles.
Les explains that by the time of Acts 28, Paul had finished his missionary journeys and was imprisoned in Rome, where he called for the Jewish leaders. He reviewed how all the Old Testament covenant promises had been presented to Israel, but they rejected them when they crucified their Messiah and left in discord and disagreement. Then Acts 28 declares, "Be it known therefore unto you that the salvation of God is sent to the Gentiles," and they will hear it, though not all will be saved. This brought a complete flip-flop: it was formerly Jew only with some exceptions of Gentiles, but now it is Gentiles with some exceptional Jews coming in. Jews can still become believers of 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.