You missed the point. Just like Males and Females are ONE IN Christ but still DIFFERENT SEXES, Jews and Gentiles are indeed ONE in Christ but still DIFFERENT. So Paul is not emphasizing the point that the Church has taken the place of the Jews calling, God says His calling is without repentance, he is emphasizing that Gentiles (Galatians) do not have to become Jew like in their observances of God in order to make it to Heaven.
If you read the whole post you see I said WE ARE ONE because we come BY FAITH ALONE, but Paul is not speaking about that here, he's speaking about just the opposite thing, Galatians trying to be like the Jews in order to make Heaven bu OBSERVING the Laws which can not save us. Thus he tells them, HEY, BY FAITH WE ARE ALL THE SAME, both Male and Female, Jew and Greek (Gentile).
To be honest, I do not see how people can't get this, males and females are NOT ONE and the same and thus neither is the Jew and Gentile on this earth, the ONLY WAY we are one and the same is how we come unto Jesus Christ/God via Salvation, by FAITH ALONE. Thus, just like we still have males and females, we still have Jews and Gentiles. Both Jews and Gentiles can only come unto God BY FAITH ALONE. Both males and females can only come unto God BY FAITH ALONE. (that was Pauls' point) A lot of people miss his point and say, but, but, but, God says the Jews and Gentiles are both one, no He is not saying that at all, God is pointing out that both can only come unto Salvation in the exact same manner, by FATH ALONE, not by the Law. Likewise, men and women who REMAIN men and women can also only come unto God by FAITH ALONE !!
Any time you see anyone emphasizing the Jews are no longer a priority with God and point to this verse, just understand Satan has TWISTED ITS MEANING with a lot of people who do not have the common sense to understand that males and females are not ONE thus Jews and Gentiles are also not ONE, it's about our Salvation being the SAME, thus those Galatians were FOOLISH to try and gain Salvation via the old Jewish practices of serving the Law. We are all ONE in that we come unto God in ONE MANNER, Jews are still Jews and Gentiles are still Gentiles, just like men are still men and women still women.