v27 – Paul quotes Isaiah 54v1. This is a command to rejoice in God’s promise. Those whose flesh has failed, who can’t produce children, are to rejoice in the knowledge that God will do for them what they can’t do for themselves. This prophecy was directed to Israelites in exile in Babylon. It seemed that the nation of Israel was finished. They’d been removed from the promised land and were being ruled over by foreigners. The situation was hopeless. But God likes to use people and situations that are humanly hopeless to show what he can do. Similarly, our attempts to make ourselves righteous are barren. Our situation is hopeless; we’ve broken God’s law and deserve his wrath, and we don’t have the ability to make things right, but we can rejoice because we’re made righteous through God’s promise.
v28-29 – In the same way that Ishmael persecuted Isaac, the Judaisers persecuted Christians. Those who rely on the law persecute those born according to the Spirit. This was a subtle kind of persecution though. The Judaisers made much of the Galatians, but it was for no good purpose. They would be friendly to the Galatians if the Galatians made much of them and followed them into slavery under the law. But in doing so the Galatians would’ve been acting contrary to the gospel, and that is unacceptable, so Paul says that, like Ishmael, the Judaisers needed to be cast out, and they won’t inherit the kingdom of God.
This makes me uncomfortable. It seems divisive to cast people out when they’re teaching something that sounds really quite similar to Christianity, and it seems really harsh that people who work so hard and are so sincere in their religiosity are not going to inherit along with those born through promise. But that’s how important it is that we reject any teaching that conflicts with the gospel of grace.