Abraham had two sons, Ishmael and Isaac.

Ishmael was born after the flesh (by Abraham’s human effort) and Isaac was born according to the promise. Ishmael was a descendant of Abraham as was Isaac, but Isaac was the child of promise.

Isaac also had two sons, Esau and Jacob.

Esau was the heir to the blessing (the promise God made to Abraham) because he was Isaac’s first born son. However, Esau did not esteem the blessing and sold his birthright for a pot of stew to satisfy his flesh. Jacob esteemed the birth right and desired the blessing. Through a series of events, Esau lost out, and Jacob inherited the blessing.

Esau was a descendant of Abraham as was Jacob, but God loved Jacob and hated Esau because Jacob desired the blessing and Esau despised it.

As it was with Ishmael and Isaac, and Esau and Jacob, so it was with the children of Israel. Within Israel, there were those who were Abraham’s descendants according to the flesh only. They were never heirs according to the promise because they never had circumcised their hearts towards God. God’s call to Abraham’s physical descendants was to circumcise their hearts and not their flesh only.

Circumcise therefore the foreskin of your heart, and be no more stiffnecked. ~ Deuteronomy 10:16

And the Lord thy God will circumcise thine heart, and the heart of thy seed, to love the Lord thy God with all thine heart, and with all thy soul, that thou mayest live. ~ Deuteronomy 30:6

Circumcise yourselves to the Lord, and take away the foreskins of your heart, ye men of Judah and inhabitants of Jerusalem: lest my fury come forth like fire, and burn that none can quench it, because of the evil of your doings. ~ Jeremiah 4:4

Because of uncircumcised hearts, all of the first generation of Israelites which left Egypt, with the exception of Joshua and Caleb, died in the wilderness. They never inherited the promised land because of their unbelief. Joshua and Caleb were children of the promise because they (with circumcised hearts) trusted God as did Abraham. The rest were only children after the flesh and died in the wilderness.

Before the second generation of Israelites entered the promised land, Joshua sent 2 spies to spy out the land. While there, Rahab who was a harlot in the land, hid them to protect them from the inhabitants of the land, because she feared the God of Israel. This act of Kindness was rewarded when Israel entered and took possession of the land.

Rahab was saved and became the wife of an Israelite named Salmon, and as a result, she became an ancestress of both King David and the Lord Jesus Christ. Rahab was grafted into Israel because of her faith and fear of the God of Israel.

Being a true Israelite was never about DNA, It was always about the heart, and having a right heart towards God.

Ruth, who married Boaz, was a Moabite woman (Ruth 1:22; 2:2, 21; 4:5, 10). Yet it was in her heart to serve the God of Israel. She said to Naomi, thy people shall be my people, and thy God, my God. ~ Ruth 1:16

Ruth, a Moabite by physical descent, also became an ancestress of King David and the Lord Jesus Christ because of her desire for God. On the contrary, we read in Numbers 25 how some Israelite men committed fornication with some unholy Moabite women and became idolatrous of the Moabites’s false gods. These men were judged by God and put to death. They had not circumcised their hearts and were not true children of Abraham, though Abraham’s descendants according to the flesh.

It has always been about the heart with God. It has never been about race or nationality. Man looks on the outward appearance but God looks on the heart ~ 1 Samuel 16:7. This is why God rejected Saul (an Israelite) and accepted David (an Israelite).

In John 8 Jesus was opposed by some religious Jews who boasted that they were the children of Abraham. Notice the exchange:

If the Son therefore shall make you free, ye shall be free indeed. I know that ye are Abraham’s seed; but ye seek to kill me, because my word hath no place in you. I speak that which I have seen with my Father: and ye do that which ye have seen with your father. They answered and said unto him, Abraham is our father. Jesus saith unto them, If ye were Abraham’s children, ye would do the works of Abraham. ~ John 8:36 -39

Jesus goes on to tell them that not only were they not Abraham’s children but they were actually the children of the devil even though they were physical descendants of Abraham.

Now, consider Paul’s explanation from Romans 9.

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 9:7-8

We can see from these words of Paul that not all in Israel were the children of the promise, and therefore were not considered the seed of Abraham in the eyes of God. This is consistent with the words of Jesus in John 8 mentioned above.

Only those who were children of the promise were counted as the seed. Who then are the children of promise? They are those who are the children of Abraham by faith, who walk in the steps of Abraham’s faith. Rahab was a daughter of Abraham even though she was outwardly a Gentile prostitute, because she feared God and in faith hid the spies.

By faith the harlot Rahab perished not with them that believed not, when she had received the spies with peace ~ Hebrews 11:31

Here is what many Christians fail to understand in their approach to the Bible: God never promised Abraham that his children would be of his physical lineage only. It was the law, which separated Israel from all other nations, not God’s promises to Abraham.God’s promise to Abraham was to make him the father of many nations.

For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith. For if they which are of the law (Israel in the flesh) be heirs, faith is made void, and the promise made of none effect: Because the law worketh wrath: for where no law is, there is no transgression. Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were. ~ Romans 4:13-17 

The promise which God made to Abraham was that he would be the father of many nations. Abraham believed this promise and was declared righteous in the sight of God.

According to Galatians 3:8, when God made this promise to Abraham, He preached or declared the gospel to him. In the fullest sense of the word, Abraham believed the gospel when he believed God’s promise to make him the father of many nations and this is how he was declared righteous. Abraham believed the gospel of Christ in advance!

Christ is the promised seed of Abraham according to Galatians 3:16. And the one through whom the blessing promised to Abraham comes to fruition. It is only in and through Jesus Christ that a person can be justified in the sight of God. Abraham was justified by God because he believed the promise God made in Christ  (Galatians 3:17) when God promised to make Him the father of many nations. This is the promise of the Spirit (the promise given through the spirit to Abraham) mentioned by Paul in Galatians. 

Being a descendant of Abraham after the flesh is not the same as being an heir to the promise.

There are those who are Abraham’s seed according to the flesh, and there are those who are Abraham’s seed according to the promise. Only those who are Abraham’s seed according to the promise are the children of God!For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise. ~ Galatians 3:26-29