“For he is not a Jew who is one outwardly, … but he is a Jew who is one inwardly; with a circumcised heart, in the Spirit, …” (Rom. 2:28-29) Under the new covenant being a Jew outwardly only isn’t considered a true Jew by God, but only those with a circumcised heart.
“For they are not all Israel who are of Israel, nor are they all children because they are the seed of Abraham; … That is, those who are the children of the flesh, these are not the children of God; but the children of the promise are counted as the seed.” (Rom. 9:6-8) Notice that national Israel are no longer considered the children of God, but only those who are in Christ (see Gal. 3:28-29)! If you are Christ’s, whether Jew or Gentile, you are a child of God!