In Genesis 17, God made a promise to Abraham and his seed. Under the old covenant the seed of Abraham was his offspring, the Jews. Under the new covenant, the seed of Abraham is no longer national Jews, but rather believers, whether Jews or Gentiles.
“Now to Abraham and his Seed were the promises made. He does not say, “And to seeds,” as of many, but as of one, “And to your Seed,” who is Christ.” (Gal. 3:16) Notice that under the new covenant the seed is Christ!
“There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise.” (Gal. 3:28-29) Only those who are Christ’s, whether Jew or Gentile, are Abraham’s seed and heirs of the promises God made to Abraham!