Ishmael was born of Hagar, Abraham’s concubine, as a result of Abraham’s accepting a proposal from his wife, Sarah. Ishmael was rejected by God because he was born of Abraham according to the flesh, that is, by man’s natural strength, the strength of the flesh, and not by God’s grace (Gen. 17:18-19). Hagar the maidservant signifies the covenant of law, which brings forth children unto slavery (Gal. 4:22, 24). Ishmael, who was born of Hagar into slavery, could not inherit Abraham’s possessions with Isaac, who was born of Sarah, the free woman. This signifies that those who keep the law by their self-effort are in slavery and are born of the flesh, and they cannot be heirs of God with the believers, who are born of grace (Gal. 4:30); that is, they cannot inherit God’s promised blessing—the all-inclusive Spirit.
When Isaac grew and was weaned, Ishmael mocked him (Gen. 21:9). Sarah, unable to tolerate Ishmael’s mocking of Isaac, asked Abraham to cast out both Hagar and Ishmael, for she would not allow Ishmael to be heir along with Isaac (Gen. 21:10). Abraham obeyed God and cast out both Hagar and Ishmael, as he was told by Sarah (Gen. 21:12-14). This signifies that the law and those who keep the law by their self-effort are cast out by grace from the inheriting of grace and are unable to enjoy grace as their inheritance (Gal. 4:30-31; 5:4).
God’s aim in calling Abraham was to accomplish His purpose, which was to establish a kingdom that He might rule on the earth and express His glory. In order to have a kingdom, seeds were needed. Therefore, God promised Abraham that he would beget a son by his wife Sarah (Gen. 17:19) to be his heir.
Isaac was the son born of Abraham by his wife Sarah, and he became the unique seed who inherited all the riches of his father (Gen. 17:19; 21:2-3; 25:5). Sarah, the free woman (Gal. 4:22b), symbolizes the new covenant of grace (Gal. 4:24a), which has the proper position in God’s economy to bring forth children as God’s heirs who will inherit what is of God. Therefore, Isaac represents the New Testament believers, who are born to be sons of God according to His grace and are His heirs who will inherit His possessions (Gal. 4:28, 30-31).
When Abraham had become old and as good as dead, and Sarah had become out of function (Rom. 4:19), according to God’s promise and at the appointed time, the time of life (Gen. 17:21, lit.), the time when the effort of the natural life ceases, Isaac was born. Therefore, Isaac was not produced by the natural strength, the strength of the flesh (Gal. 4:23), but by God’s strength in grace. He was born not of blood, nor of the will of man, nor of the effort of the flesh (John 1:13), but according to the Holy Spirit (Gal. 4:29), who is God reaching man (cf. Gen. 17:21—21:1). Therefore, Isaac represents the New Testament believers, who are born according to the grace of God’s promise—those who are not born of the flesh but of the Holy Spirit.