Christians aren't "true Israel." Descendants of Abraham, Isaac and Jacob who have the faith of the same kind as Abraham's are "true Israel." But as to the grafting-in part, yeah ... Paul taught that in his letter to the believing assembly in Rome.
But let me ask you this. Would you consider an atheistic Jew a member of "true Israel?" That is to ask, do you consider someone who denies the existence of the God of Abraham, Isaac and Jacob to be a part of "true Israel?"
So, Christians are second-class groupies?
But let me ask you this. Would you consider an atheistic Jew a member of "true Israel?" That is to ask, do you consider someone who denies the existence of the God of Abraham, Isaac and Jacob to be a part of "true Israel?"
My understanding as far as Judaism is concerned: to the extent that he is observant of the mitzvot, yes. Even an atheist Jew can be acceptable to God. I would say as long as he did not worship false gods and lived a righteous life, he would is a part of true Israel.
You have to remember that an atheist Jew is still a Jew. A Jew who is a Christian is no longer a Jew, the way a nonpracticing Catholic is still a Catholic by baptism, but a Catholic who became an Mormon is no longer a Catholic, because he believes in an altogether different deity.