Isaac and Ishmael in Galatians 4

Genesis 21 describes the miraculous conception and birth of Isaac and the sending away of Hagar and her son Ishmael. The apostle Paul saw in Genesis 21 an analogy or foreshadowing of the situation in his own day. He describes this in Galatians 4:21-31. There he contrasts two groups personified by the two sons of Abraham:

• The son of the slave woman, born according to the flesh: "the present Jerusalem." The earthly Jerusalem was still under the law and in slavery with her children. Those who reject Christ and rely on works of the law are relying on the flesh and proving to be slaves not sons.

• The son of the free woman, born through promise: "the Jerusalem above." The heavenly Jerusalem is free and she is our mother. Those who receive Christ are children of promise, born according to the Spirit, and thus sons and heirs and free. 

Under the old covenant, even the true sons were under the bondage of the ceremonial law, as sons under age (Gal. 3:23-24, 4:1-3), but with the coming of Christ, one either progressed to the new covenant and the freedom of mature heirs or clung to the old covenant without Christ as a way of works-based salvation, proving to be slaves and not heirs. 

The Galatian saints were being tempted by false teachers to return to bondage by receiving circumcision and to rely on their works, but Paul exhorted them to stand fast in the freedom of sons through faith in Christ.

Christ is the promised offspring (Gal. 3:16), and those who are in Christ are children of the free woman, of the spiritual Jerusalem, sons and co-heirs with Christ.  “Now you, brothers, like Isaac, are children of promise” (Gal. 4:28).

Even as Ishmael persecuted Isaac (Gen. 21:8-9), so Paul could point to the persecution of Christians by the unbelieving Jewish establishment of that time (Gal. 4:29). It is common in general for those who are of the flesh to persecute those who are of the Spirit, just as within the believer there is war between the flesh and the Spirit (Gal. 5:16-26). But who will remain and inherit the covenant blessings? What does Scripture say? Paul quoted from Genesis, “Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman” (Galatians 4:30).

Therefore, do not trust in the flesh and do not rely on works of the law. Believe in Jesus Christ and the provision of grace. For it is through faith in him that one is justified and adopted as an heir (Gal. 2:6, 3:24-26).