But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now (Gal. 4. 29).
“Here the apostle brings in a further detail supplied by the allegory which was germane to his subject. He refers to the opposition made against Isaac by the son of Hagar, recorded in Genesis 21:9. This received its counterpart in the attitude of the Judaizers toward Christians. They who still adhered to the old covenant were hostile to those who enjoyed the freedom of the new. Probably one reason why the apostle mentioned this particular was in order to meet an objection: How can we be the “children of promise” (God’s high favorites) seeing we are so bitterly hated and opposed by the Jews? The answer is, No marvel, for thus it was from the beginning: the carnal have ever persecuted the spiritual.
“Nevertheless what saith the Scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the free woman” (v. 30). Here is the final point in the allegory (taken from Gen. 21:10, 12) and which incontestably clinched the apostle’s argument that Israel after the flesh are finally set aside by God. Hagar represented the Sinaitic covenant and Ishmael its carnal worshipers, and their being cast out of Abraham’s household prophetically signified God’s setting aside of Judaism and the fact that the natural descendants of Abraham had no place among his spiritual children and could not share their heritage (cf. John 8:34, 35). The two cannot unite: pure Christianity necessarily excludes Judaism. In its wider application (for today): none who seek salvation by law keeping shall enter heaven.” – A. W. Pink