On what basis are both Jews and Gentiles saved?

Les explains that in the Old Testament Gentiles were strangers from the covenants of promise, aliens from the commonwealth of Israel, without hope and without God in this world. But now, since the gospel of grace came along, both are on a level playing field; the Jew has no advantage over the Gentile. Verse 30 says it is one God who shall justify the circumcision by faith and the uncircumcision through faith. Les says the difference between “by faith” and “through faith” is none; it is the same thing. A Jew is saved on the same basis as a Gentile, and that is faith in the finished work of the cross. There can be nothing added to it.