“For it is written (in the law) that Abraham had two sons”. Paul had mentioned Abraham several times back in chapter 3. It seems that the false teachers had been saying that to be counted as sons of Abraham you needed to keep the Old Testament ceremonial law and be circumcised like Abraham was. Now Paul points out that Abraham had two sons, and he explains to the Galatians that yes, the false teachers are showing you how to be sons of Abraham, but they’re teaching you how to be slaves like Ishmael, not children of the promise like Isaac.
Isaac and Ishmael illustrate the gospel. Abraham and Sarah were both very old, and Sarah had always been barren. It was impossible for them to make a baby. So they decided to help God out with his promise to give them a child. Abraham would have a child through Hagar rather than Sarah. So Ishmael was born. Ishmael was the result of Abraham’s attempts to fulfil God’s plan by his own works. And in Genesis 17 Abraham said to God “Oh that Ishmael might live before you!” but God said “No, but Sarah your wife shall bear you a son, and you shall call his name Isaac. I will establish my covenant with him”. Similarly, God rejects our attempts at making ourselves righteous; he’s promised to make us righteous through Christ.
Since Ishmael didn’t become Abraham’s heir, he inherited the status of his mother, Hagar, as a slave. When Abraham attempted to fulfil God’s plan by his performance in the flesh, the result was a child born into slavery. And Paul says this illustrates the covenant at Mount Sinai. God gave Israel the law at Mount Sinai, and as we saw at the beginning of chapter 4, it enslaves people; that’s part of what it was designed to do. Until the date set by our father, we were under a guardian, no different from a slave. But then, as promised, God sent his son to redeem us from slavery to the law, and to adopt us as his sons and daughters. But the present Jerusalem had rejected Jesus and instead chosen to remain in slavery.
Paul started off this section in v21 by asking “do you not listen to the law?” So I think when he goes on to compare Ishmael and Isaac he’s kind of posing further questions… Which is your spiritual home: the present Jerusalem which is full of people trying to make themselves righteous by their works of the law, or the Jerusalem above where God dwells? What is your relationship with God based on? How well you’ve kept your end of the Mount Sinai covenant, or God’s unconditional promise to Abraham? Were you born according to the flesh or the Spirit?