Galatians 4:28

Now we, brethren, as Isaac was, are children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, so also it is now.

Here the reference is to the event of Genesis 21:9ff. The enmity between these two branches of Abraham's family has continued until the present day; and there has also been a corresponding hatred of the secular, carnal, fleshly and unspiritual against the holy teachings of Christ also. Paul intends for the Galatians to see that the Judaizers are actually their enemies, having no good thing for them, at all, in their purposes.