Know then that it is those of faith who are the sons of Abraham. 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.” So then, those who are of faith are blessed along with Abraham, the man of faith.
This is a tough one, especially if you are a Jew, but also if you tend toward legalism. The "sons" of Abraham were thought to be the Jews, or those who obeyed the Jewish law. But God knew, and in the Scripture he revealed, that the true sons of Abraham would be sons by faith.
The Scripture revealed that in Abraham, ALL the nations would be blessed. And by faith, some from all nations shall be redeemed at sons of Abraham. This is a challenge for the Jew, many of whom would say that if you are going to save the gentile sinners, then leave me out. And God will do just that.
But there are those among the gentiles who act just like the Jews, thinking that they now own the birthright, and God owes them a place in heaven. Same kind of thinking that the Jews held to. (See definition of irony).
Abraham trusted God, and it was credited to him as righteousness. The Jews and legalists trusted themselves and their understanding of righteousness. But that is kind of like trusting a fraying rope to get you up a mountain. When it breaks, you have no one to blame but yourself.