QUESTION : But surely Jews who become Christians are not Jews any more?
I hope, David, that, by now, this question has been answered.
If not, then let me be clear about it, Jews who believe in Jesus are still Jews! Let's be sensible here. According to the Hebrew Scriptures a Jew is a descendant of Abraham, Isaac and Jacob. Someone who is born a Jew, also dies a Jew.
It doesn't matter what they believe in during their life, nothing can take away their Jewishness. Perhaps this position would be a little easier for you to accept if you consider that Christianity was originally intended to be a very Jewish faith.
Jesus is Jewish, as well as virtually all of his early followers and the New Testament writers. He died a Jew, the word 'Christian' hadn't been invented yet, so why is it possible for his followers to lose their Jewishness?
I consider myself a completed Jew, in that Jesus totally fulfilled the Torah (I've covered this in more detail elsewhere). Gentiles who accept Jesus, though they don't become Jews, they are grafted into the Jewish olive tree of faith, becoming the children of Abraham by faith, if not physically.
As for me, I'm a child of Abraham spiritually and physically. You too can add your true spiritual heritage to your physical one, simply by accepting Jesus as Messiah.