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.
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.