Romans 11:13 Inasmuch as I am an apostle to the gentiles, I glorify my ministry in order to make my own people jealous—and thus save some of them.

Paul was a Jew himself, and always trying to figure out ways to bring his own Jewish brothers and sisters to Christ. This was just one of his many attempts: show them the wonders of GENTILES who had come to believe. What would make US jealous enough to get us serving and believing?