Galatians 4:28

Now we, brethren, as Isaac was, are the children of promise. Since he was born of one barren through age not according to the flesh, but according to the promise of God. Ver. 29. He that was born after the flesh. Ishmael, born naturally of Hagar, persecuted Isaac, born supernaturally of Sarah, according to the Divine promise, and so a type of the spiritual children of the New Law. The reference is to Gen. xxi. 9. From a comparison of these two passages it is evident that the mockery mentioned was a sort of persecution, the sort of sport that cats have with mice. So in 2 Samuel 2:14 : "Abner said to Joab, Let the young men now arise and play before us," where the play was a mortal combat. Jerome and others think that the reason why Ishmael persecuted Isaac was because his envy was stirred up by the festivities indulged in at Isaac's weaning, and because he was jealous of the birthright assigned to his brother by promise. Hence it appears that he was hostile to the promised Seed, i.e., to Christ.

So it is now. As formerly Ishmael mocked and persecuted Isaac, so now have the Jews mocked and crucified Christ, the King of liberty, and are still pursuing with bitter hatred His followers. So too are they persecuting you, 0 Galatians, that they may enslave you, and turn you from the right way. See the comments of Jerome and Rupert on Genesis 21:9.