7 Neither, because they are the seed of Abraham are they all children: but, in Isaac shall thy seed be called.
Here Paul quotes Genesis 21:12-13:
But God said to Abraham, "Do not be distressed because of the lad and your maid; whatever Sarah tells you, listen to her, for through Isaac your descendants shall be named. And of the son of the maid I will make a nation also, because he is your descendant. And also of the son of the bondwoman will I make a nation, because he is thy seed."
This is the context: after Isaac had been born, Sarah wanted Abraham to throw out Ishmael and make Isaac his heir. Abraham at first worried about this, but God said go ahead and do it, Isaac will be your heir and so the nation of Israel will be established through him. To comfort Abraham, God also let him know that Ishmael would be fine and would become a nation. This is God’s reaffirmation of His original promise to make Abraham a great nation. God is revealing a bit more about how He will fulfill His promise.
Physically, God’s choice was to make Isaac and not Ishmael Abraham’s heir. Paul is using this example of a son of Abraham who was rejected. The Jews are sons of Abraham who are rejected. So God’s choice does not bless people just because they are Israelites.
Paul teaches a deeper spiritual truth though this example. In the passage in Genesis, at a spiritual level, Isaac represents the Gospel and Ishmael the law. (Galatians 4:22-31) God chose to save through the gospel, not the law. The children of Abraham are those that believe. (Galatians 3:6-7, 39) This answers the Jew’s question as to why they are rejected. It was always God’s plan to reject unbelievers, even Jewish ones who earnestly follow the law. The Jews, who were attempting to gain salvation through the law are Israel (physically), but not of Israel (spiritually).
The concept of “calling” (Greek κληθησεται), or as it can be translated “naming”, is important. The idea is not one of invitation, but rather designation. A group was to receive a special designation by God, a designation which would set them apart from other people. Physically that was the nation of Israel, God’s chosen people. Spiritually, God calls believers His children. The concept of naming ties this declaration back to God’s original promise to Abraham that he would become the father of a great nation. Isaac and his offspring were to be identified as the great nation God established through Abraham.
It is important to see this as a three step process. First, God gives a promise. Second, people believe God’s promise. Then third, God fulfills His promise. The promise to Abraham was “I will make you a great nation and in you all the nations of the world will be blessed”. Then Abraham believed God’s promise. Finally, God made Abraham a great nation. Implicit within the promise to Abraham is the Gospel. God would send Christ through Abraham to bless the world. When Abraham believed God, God justified Abraham. Today that same promise given to Abraham is even more explicit. God promises that whoever calls upon the name of the Lord will be saved. Then people believe. Finally, God saves them.
8 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.
Physically, not all of Abraham’s children (the children of the flesh) were blessed. Isaac was; Ishmael was not. This removes the objection that descent guarantees blessings.
Spiritually, Paul distinguishes between the children of the flesh (Jews who follow the law) and the promise (those who have faith). The flesh represents those who through human endeavor follow the law. (Galatians 3:2-3) The children of the flesh are those who were seeking salvation through their own strength using the law. Not all national Israelites are rejected or children of the flesh, Paul himself being a prime example. In other words, Paul is subdividing Israel into spiritual Israel (the children of the promise) and the Israel that is rejected (the children of the flesh).
The children of the promise are those who follow Christ through faith. (Galatians 3:8) They, as Abraham, believe God’s promises and are declared righteous and adopted into God’s family as His children. (Romans 4:1-5) God chose to give the promise to Isaac, not Ishmael. Just because they are Abraham’s children by nature does not make them God’s children.
Verse 8 summarizes Paul’s argument from verses 6 and 7. Paul removes the false notion regarding national Judaism. The statement here in verse 8 “they which are the children of the flesh, these are not the children of God” corresponds to his earlier statements: “for they are not all Israel, which are of Israel” and “neither, because they are the seed of Abraham are they all children”. Paul denies firmly that national Jews are: 1) Spiritual Israel 2) Abraham’s spiritual children and 3) the children of God. Thus the Jewish argument that nationality brings them within God’s covenant is false. Being a national Jew is insufficient.
9 For this is the word of promise, At this time will I come, and Sarah shall have a son.
Here Paul quotes from Genesis 18:10:
He said, "I will surely return to you at this time next year; and behold, Sarah your wife will have a son." And Sarah was listening at the tent door, which was behind him.
God had already promised that He would make Abraham a great nation. Later God would explain to Abraham that Isaac would be his heir. But in between, He tells Abraham that Sarah, who is barren, will have a son. Not only did God confirm His plan but He clarified the promise. Each clarification reveals more and more to Abraham about how he will be made a great nation.
Promise, in the Greek, is genitive. It’s literally the promise’s word or the word the promise possesses. The point is that these words (i.e. that Sarah will have a son) are based on the original promise to Abraham that God would make him a great nation and through him all the nations of the world would be blessed.
Physically, Isaac would become the father of the nation of Israel. Through him the law and promises would be passed and God would work with the nation of Israel. God chose to work through Abraham and make him a great nation. God could have chosen Ishmael, the firstborn, to be the nation of Israel and Abraham’s heir, but He did not. So the Jews cannot count on the fact that they are Abraham’s natural children, because it does not guarantee blessings.
Paul is also teaching a deeper spiritual truth. Isaac was born blessed with being in the lineage of Christ because of the promise. The foundation of the blessing was not the flesh, but God’s promise. The promise to Abraham was more than that he would have a son, but it is the promise of the Gospel. (Galatians 3:22)