As Jesus talked with Nicodemus, He said, “‘I tell you the truth, no one can see the kingdom of God unless he is born again.’ ‘How can a man be born when he is old?’ Nicodemus asked. ‘Surely he cannot enter a second time into his mother’s womb to be born!’ Jesus answered, ‘I tell you the truth, no one can enter the kingdom of God unless he is born of water and the Spirit. Flesh gives birth to flesh, but the Spirit gives birth to spirit. You should not be surprised at my saying, “You must be born again”’” (John 3:3-7).
The phrase “born again” literally means “born from above.” Nicodemus had a real need. He needed a change of his heart—a spiritual transformation. New birth, being born again, is an act of God whereby eternal life is imparted to the person who believes (2 Corinthians 5:17; Titus 3:5; 1 Peter 1:3; 1 John 2:29; 3:9; 4:7; 5:1-4, 18). John 1:12, 13 indicates that being “born again” also carries the idea of “becoming children of God” through trust in the name of Jesus Christ. (https://www.gotquestions.org/born-again.html)
Galatians 4:21-31 explains in greater detail the idea of being born again and how it relates to the foreshadowing in the Old Testament scriptures (the Decalogue).
The Scriptures make it clear that just as Hagar and Ishmael were cast out of the camp of Abraham, so will all those who are NOT born again be cast out. (Genesis 21:9-14)
Verse 21 – We are reminded to refer back to what the law, written by Moses, has to say about the two dispensations.
Verse 22 – Abraham had two sons: one by the bondwoman Hagar (Genesis 16:15) and one by the freewoman Sarah. (Genesis 17:15-16)
Verse 23 – The son of the bondwoman was born from a fleshly (manmade) plan. The son of the freewoman was born from a spiritual (God-made) plan.
Verse 24 – These two sons represent the two covenants of God – the first covenant of the Old Testament (the Law) with the nation of Israel. The second covenant of the New Testament (the Spirit) with all those who come to faith in Christ.
Verse 25 — The bondwoman represents the Jerusalem that now exists on earth – the mother of those who trust in the law and are in bondage.
Verse 26 – The freewoman represents the Jerusalem of Heaven – the mother of us all (who come to faith in Christ). This is the mother of those who are born again – born of the Spirit.
Verse 27 – We are told in this verse that the Jerusalem of Heaven has more children than the Jerusalem that now exists.
Verse 28 – Continuing the allegory, the Scripture reminds us that we who are born again are children of the promise of God – an allegory to Isaac who was Abraham’s child of promise.
Verse 29 – And just as Isaac, the child of promise, was persecuted by the child who was born of the bondwoman, so we who are the children of promise (born again) are persecuted by those who are not. We are the children of promise and guaranteed entry into the eternal kingdom of God, joint-heirs with Christ.
Verse 30 – We are reminded again of the allegory of Abraham – that just as the bondwoman and her son was cast out of the camp, so those who are not born again will be cast out. The reason given is that the bondwoman’s children shall not be heirs with the freewoman’s children. We are saved by faith in Jesus Christ and as such become heirs according to the promise (Galatians 3:29). There is no work of the law that can save anyone. There is no good deed that can save. In fact, there is no LAW that leads to life! (Galatians 3:21)
Verse 31 – So then we who are children of the freewoman are children of Jerusalem of Heaven.