‘But as then he who was born after the flesh persecuted he who was born after the Spirit, even so it is now. But what does the Scripture say? “Cast out the handmaid and her son, for the son of the handmaid shall not inherit with the son of the free-woman”.'
But significantly, just as initially the one born ‘of the flesh' persecuted the one born according to the Spirit, so it still is. The true Christians are being ‘persecuted' by the Judaisers and by the Jews. The Scripture therefore says they are to cast the Judaisers out, and have nothing to do with them, for it says, ‘Cast out the slave and her son, for the slave's son shall not inherit with the free woman's son'. So as we are not children of the slave woman, but of the free woman, we should have nothing to do theologically with those who are still slaves to the Law.
Paul has in mind the Judaisers, but the message also applies to all who seek to bring Christians into subjection to ordinances and regulations. Once these become seen as part of that which is necessary for salvation and Christian growth (rather than as just helpful) those who promulgate them must be cast out and removed.
We notice here how the argument has moved from just being ‘under Law' and ‘under promise', to being ‘of the flesh' and ‘of the Spirit' in readiness for what is to follow. To be under the Law is to live according to the flesh. To live truly under promise is to live according to the Spirit.
Furthermore there is another interesting result of his argument. It demonstrates that those who called themselves the sons of Abraham and saw themselves as the true sons of Abraham were not true sons of Abraham. In God's eyes they were spiritually Ishmaelites. They would no longer inherit the promises promised to Abraham's true seed. They were no longer the true Israel, the Israel of God. It was the church who were the true descendants of Abraham through Isaac. It was they who were the true Israel of God (Galatians 6:16). The unbelieving Jews were to be seen as descended through Ishmael. They had been cut off from the olive tree (see Romans 9:6-8; Romans 11:19-20).