The meaning of the two sons in this passage is absolutely basic to all spiritual wisdom. If we listen to carnal whispers will get this all tangled up—and we must not.
Tell me, ye that desire to be under the law, do ye not hear the law? For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar. For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. But Jerusalem which is above is free, which is the mother of us all. For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband. Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. So then, brethren, we are not children of the bondwoman, but of the free (Gal. 4:21-31).
Those Galatians who desire to be under the law have this problem—they are refusing to listen to the law (v. 21). Abraham had two sons. Ishmael was born of the concubine Hagar, and Isaac was born of the free wife, Sarah (v. 22). The son of the slave woman was after the flesh, but the son of Sarah was the fulfillment of promise (v. 23). Now, Paul says, messing with our modernity, this is an allegory. Sinai corresponds to Hagar, which corresponds to Jerusalem below, all of which tend to spiritual bondage over generations (vv. 24-25). But the heavenly Jerusalem is free, and is our spiritual mother (v. 26). Paul quotes Isaiah 54 here. The barren woman is now more fruitful than the other woman (v. 27). We Christians, Paul says, are like Isaac—children of promise (v. 28). The analogy is widereaching. There is antipathy between the two children (v. 29). The children of the flesh are to be disinherited; they cannot be joint heirs (v. 30). Christians are the children of Sarah, not Hagar (v. 31).
Allegory causes us problems. In the late seventies, I took a summer course in hermeneutics from a well-known evangelical seminary. Our project was the book of Galatians, and when we got to this portion of the text, the instructor said that Paul was wrong to do what he did here. Allegorical interpretation was forbidden. While many would see this as extreme (and heretical), we still tend to share this gentleman’s suspicions. If we interpret the Bible "allegorically," where are the brakes? What is to prevent us from careening off into a very baroque school of interpretation? There are two responses here. One is that we assume that our current mode of "interpreting" doesn’t need brakes. And second, we have forgotten how much Scripture teaches us about types and allegories.
We need biblical eyes. The issue is not this particular passage or that one. The issue is not whether our handling of a passage could be tighter or not. The issue is how we think, how we respond. Do we let Scripture define everything in our lives, or do we try to have our lives (and our understanding) shape the Scriptures?
The Church is our mother. Sarah was a type of the Christian church. Her long years of barrenness correspond to her time in the Old Covenant. She was the wife of Abraham, but she was barren. Paul argues that the Jerusalem above is a woman who used to be barren, and who now has a multitude of children. She is the free woman—but there was a time when the free woman appeared to be fruitless and the slave woman appeared to be fruitful. The Church above is our mother. The heavenly Jerusalem is not built on a mountain that can be touched by human hands (Heb. 12: 18ff). Come, the angel said, I will show you the bride, the wife of the Lamb. And John was shown the new Jerusalem, descending from heaven (Rev. 21:9-10). This is the Church, this is our mother.
And as we learn to relate to our mother, we will always be dealing with two kinds of children. There will always be Isaacs, and there will always be Ishmaels. This transition from the Old Covenant to the New is the point where Sarah has borne her freeborn son, and the slave woman is divorced and put away. Now are there no more temptations or pitfalls? Of course not. Until the world ends, there will always be those who live carnally in unbelief and those who live in faith, by faith, and unto faith. Over time, the ratios between these two groups change, but we are always dealing with them. But not only so, we will always be dealing with them within the visible Church. The new covenant is not the time when it becomes impossible to be an Ishmael.
We are promised that the children of the free woman will overrun the earth. Abraham was promised the world (Rom. 4:13), and his (free) children will possess it. And five years before the Last Trump, there will be some poor fool still clutching at his unbelief. This text summons us and commands us to be unlike that man. The just shall live by faith, and the meek will inherit the earth.