Paul wants his readers to understand that since they have put their confidence in Christ, rather than in a religious system, they too are not of Ishmael but of Isaac. Galatians 4:28 says, “Now you, brothers, like Isaac, are children of promise.” There are several contrasts between the child of Hagar and child of Sarah. The child of the slave girl, Ishmael, came into the world through natural means. Abraham had sexual intercourse with the slave girl provided to him by his wife Sarah and she conceived and delivered a son. Nothing at all out of the ordinary. This is all completely natural and was the result of the “will of man,” not according to the will of God. Sarah’s son Isaac, on the other hand, was born of a supernatural birth according to the will and promise of God. His conception and birth were God’s promise and not man’s decision. You might also notice that Ishmael was circumcised at the age of 13, the age of awareness, while Isaac was circumcised as an 8 day old infant, an age at which a person is not even conscious of what is taking place around him or the significance of it. In other words, Ishmael represents the rational, legal, and natural relationship with God, while Isaac represents the supernatural relationship.
There are two kinds of births. The first is physical, and every human has a birthdate that marks this event. Then there is the spiritual birth, or as Jesus informed Nicodemus, a “re-birth.” When Jesus said you must be born again, He was referring to the spiritual birth that takes place once a human puts their faith in Jesus and not in religion. One is flesh and one is spirit. One is supernatural, one is natural. The first makes us children of God by creation. The second makes us children of God by redemption. Those redeemed are the spiritual heirs and the legitimate children of God. The others remain slaves according to the flesh.
From the very beginning, there has been animosity between the two children. Galatians 4:29 goes on to clarify the perpetual dynamic that takes place between the flesh and the spirit. Paul writes, “But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.” Conflict between the two sons of Abraham began from the very start. This jealousy and resentment between Sarah and Hagar and their sons Isaac and Ishmael created an unparalleled hate which has set off wars and atrocities for four thousand years. What Paul is saying here is that those who were trying to pull the Galatian believers back into an observance of the laws were making themselves like Ishmael, while the believers in Christ and God’s grace were like Isaac. We, like Isaac, relate to God based on who we are. We do not relate to God like Ishmael, based on what we do!