Galatians 3:7-9 ESV
[7] Know then that it is those of faith who are the sons of Abraham. [8] And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.” [9] So then, those who are of faith are blessed along with Abraham, the man of faith. 

Galatians 3:16-18 ESV
[16] Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. [17] This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. [18] For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise.

Galatians 3:28-29 ESV
[28] There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. [29] And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.

In other words, Paul's point of saying there is now no distinction between Jew or Greek is because the promises made to Abraham are only applicable to those who believe in Christ. So neither Jewish descent or Gentile descent matters. Christians, whether descended from Jews or any other nationality, are the only legitimate heirs of the promises made to Abraham and his seed.