Does a Jewish believer have to give up being Jewish?
No. A Jewish believer does not have to give up his Jewishness. However, he must separate himself from the Old Testament economy, because this is the age of grace and believers have nothing to do with the Law. That was all settled at the cross.
Les explains from Romans chapter 10 that Israel is being dealt with in this present dispensation, the age of grace. As Paul made plain in Romans chapter 3, since everything was accomplished at the cross and the gospel is now going out into the Gentile world, there is no difference between Jew and Gentile. A Jew does not have to give up his Jewishness, but he must separate himself from the Old Testament economy. This age of grace has nothing to do with Law, because that was all settled at the cross. Les points to Colossians, where Scripture says He took the Law and nailed it to His cross.