Paul emphasizes that these Jewish and Gentile believers are "one body." And the way that could possibly happen was because of what Christ did on the cross. He made them right with God and, thus, made them right with each other. He ended all "hostility" and prejudice the Jews and Gentiles used to have for each other. Now they can live a life of peace and unity together. You might have to go back to Acts 15 to see how the early church dealt with receiving believing Gentiles into their fellowship. This was a radical experience for the Jews as well as a overwhelming surprise to the Gentiles!
Let's keep in mind that the ending of hostility wasn't just between Jew and Gentiles, it was also between God and the believing sinner! No longer are we enemies of God - we are now His children and His friends!