In the book of Romans the apostle Paul makes a distinction between the Israelites by telling us, “…they are not all Israel, which are of Israel. Neither, because they are the seed of Abraham, are they all children: but, in Isaac shall thy seed be called. That is, they which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.” ~ Romans 9:6b-8
Paul applies the promise God made to Abraham which says “in Isaac shall thy seed be called” to those who are the children of God by faith. These are the true children of Abraham in the eyes of God, and not those who were of natural descent only. Scripturally speaking, those who are children of Abraham (in truth) are the children of God.
Paul uses similar language in Galatians as in Romans when he says to his Gentile brethren, “if ye belong to Christ, then are ye Abraham’s seed, and heirs according to the promise.” ~ Galatians 3::29. He also says, “For we, brethren, as Isaac was, are the children of promise.” ~ Galatians 4:28
Both of these are references to being children of God by faith and heirs of the promise which God made to Abraham. Abraham is the father of all who believe and that is why Paul makes the distinction that he does between the Israelites who sought righteousness through faith and those who sought it through the works of the Law.
In Romans 9-11, Paul tells us that only a remnant of the people of Israel truly found God’s way of righteousness and that the people of Israel, as a whole, did not obtain righteousness because they did not seek it by faith. Instead, they sought it by the works of the Law: through their national identity.
In Romans 4, Paul tells us that because of Abraham’s faith, he was declared righteous in the sight of God, apart from the works of the law. Abraham believed God and it was accounted to him as righteousness. In Galatians Paul says, “…the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, in thee shall all nations be blessed.” ~ Galatians 3:8
Abraham believed the Gospel in advance which was God’s promise to justify all who believe. Therefore all who are of faith (faith in Jesus) are the children of Abraham and heirs to the promise.