We look at Genesis 21, which is the basis for Paul's thoughts in this passage. In Genesis 21, Isaac ('laughter') is born, speaking of the rejoicing in Heaven whenever someone in born again. When Isaac was weaned, there was a feast in his honour (v8), which resulted in Ishmael manifesting his insecurity by mocking Isaac (v9). Ishmael, who was 14 years older, was used to being in the ascendency, but now his position was threatened, by Isaac, the child of promise. Paul says this is a picture of how the children of the flesh always persecute the children of the Spirit, and we give examples of this from history. This is also a picture of how the work of the flesh resists the promise of God and the work of the Spirit in our own lives today. Sarah realised that because of this antagonism, Hagar and Ishmael had to be cast out in order for Isaac's promised inheritance to be established, and God agreed with her (v10-12). Likewise, the flesh and the Spirit are mutually exclusive, like bondage and freedom. They are always in opposition (Galatians 5:16-17), and cannot coexist. For the work of the Spirit to be established in our lives, like Abraham, we must cast out the works of the flesh. Otherwise, they will continually resist the fulfilment of God's promise and the work and fruit of the Spirit in our lives. One application to the Galatians is that although the legalistic false teachers were physically Jewish, they were spiritually like Ishmael, bound to Hagar (the Law). Therefore, they were hindering the work of God in the assembly, and so the Galatians should cast them out.

Abraham's act of obedience in casting out Ishmael and establishing Isaac as his seed (at the age of 5) marked a significant step forward in God's plan for his life (compare Genesis 15:13 and Acts 7:5 with Exodus 12:40-41 and Galatians 3:17). We know that God revealed this to him, because soon afterwards, he called on the name of 'El Olam', 'the Everlasting God' or 'the God of the Ages' (v33) - the first time this Name of God is revealed. Likewise, when we are obedient to cast out the work of the flesh, the Lord of the Ages, will move our life forward into a new season. Paul concludes that as born again believers, we are by nature free - our spirits have been set free from sin by the indwelling Holy Spirit, and we are free from the Law and its condemnation and curse (4:31).