After Abraham realized that God is the Father through his intercession for the women in Abimelech's house, Sarah bore him a son at God's appointed time. Abraham called his son Isaac. When his son was born, he was a hundred years old (Gen. 21:5).
The day Isaac was weaned, God spoke a word through Sarah: "Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac" (v. 10). This was not Sarah's jealousy. Galatians 4:30 shows us that this was God's word through the mouth of Sarah: "For the son of the maidservant shall by no means inherit with the son of the free woman." This means that only one person could fulfill God's goal. This one was Isaac, not Ishmael. Ishmael was the first, not the second. Therefore, he represented Adam, not Christ. "But the spiritual is not first but the soulish, then the spiritual" (1 Cor. 15:46). Those who are of the flesh cannot inherit the kingdom of God; they cannot fulfill God's goal. The second one was Isaac. Therefore, Isaac represented that which is spiritual, he who can inherit God's inheritance and maintain God's testimony.
It is interesting to see that Ishmael was not cast out the day Isaac was born. He was cast out only after Isaac was weaned. Without Isaac, it was impossible to cast out Ishmael. Some brothers and sisters are full of fleshly works and a fleshly walk. When they hear about the flesh and what it means, they dare not do anything any longer, and they cease their work altogether. They do not have Isaac yet. Therefore, when they cast out Ishmael, they cannot do anything. Many Christians have been used to doing things by themselves and according to their fleshly strength. When they stop their fleshly work, they are left with no spiritual work. They did not have anything spiritual before; all they had were fleshly things. When the flesh is stopped, nothing spiritual remains. The principle is that Isaac must be weaned. This means that Ishmael can only be cast out when Isaac is strong enough to be the son and has gained the proper ground.
What does it mean to be cast out? Let us read Galatians 5:1. "It is for freedom that Christ has set us free; stand fast therefore, and do not be entangled with a yoke of slavery again." This means that the Lord Jesus has freed us; He is already living within us. The life we have received is a life of freedom; we have been freed. Therefore, we should not try to do anything to please God. Whenever we try to do anything, we become Ishmael. Whenever we stop trying, we are living in the freedom of the Son. The Christian life hinges on whether or not something has been done by us. Whenever we try to do something to please God, the self and the law of sin and of death come, and we fall back to Ishmael's position and become the sons of a maidservant. As a son of the free woman, there is no need for us to do anything by ourselves. We have a life within us, and this life will do everything in a spontaneous way. We are Christians; we do not need to act out our Christian life. We are children; we do not need to act like God's children. We live in what we are, not in what we do. Whenever we try to do something, we are "entangled with a yoke of slavery again" and become sons of the maidservant. If we stand on the ground of Isaac, the life we have will spontaneously be manifested through us.
After Abraham cast out Ishmael, even Abimelech, who had rebuked him once, came to him and said, "God is with thee in all that thou doest" (Gen. 21:22). The root of failure was removed from Abraham, and God was able to fully manifest His own work through Isaac.