'Born again', 'born from above' is a very expensive metaphor. And there's no record of Jesus giving Nicodemus an explanation of it. Plus there's a whole load of baggage that the phrase has picked up in last 100 years or so, which isn't always helpful.
In the context of the original conversation between Jesus and Nicodemus, what was Jesus communicating, and why? Was Jesus being deliberately obtuse, or was Nicodemus just slow to catch on?
I don't myself have satisfactory answers for these questions.
@Butch5 do you?
I'm glad you see the phrase is a metaphor. I think I do have a satisfactory answer. God had made certain promises to Abraham. He told him that He would, make him a great nation, the father of many nations, that all nations would be blessed through him, and that He would give him all of the land he could see. This is where the Jews "Promised Land" comes from. When God made that promise He said He would give the land to Abraham and his seed. The Jews {Israel) are the seed of Abraham. Paul even acknowledges this.
2 That I have great heaviness and continual sorrow in my heart.
3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh:1
4
Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and
the promises;1
5
Whose are the fathers,
and of whom as concerning the flesh Christ came, who is over all, God blessed for ever. Amen.
(Rom. 9:1-5 KJV)
Paul says that the promises are the father's and belong to Israel. Note also that he said it was because of these that Christ came. One of those promises is the land. The Israelites believed that they were entitled to the promises because they were the seed of Abraham. The promise was to Abraham and his seed. Nicodemus was an Israelite, the seed of Abraham and believed he too was entitled to this land. The Jews were waiting for their Messiah who would conquer and rule on David's throne. This is the kingdom they were looking for. This is the kingdom Nicodemus was waiting for. The Messiah's kingdom. Nicodemus believed, as the seed of Abraham he had access to this land/kingdom. When he encounters Jesus, Jesus corrects his thinking, his birth as the seed of Abraham is not sufficient to gain him access to the kingdom. He needs another birth, a second birth, to be born again. But, didn't God say that the promises were to Abraham and his seed? Aren't the Israelites Abraham's seed, through their physical birth from Abraham? Yes, they are. So then aren't the promises to the Israelites? No, they are not. But God said, 'and your seed'. Jesus says that which is born of flesh is flesh. In other words, no Nicodemus, your physical birth as the seed of Abraham is not sufficient to gain you access to the Kingdom. Being the physical offspring is not going to get you into the Promised Land. Jesus tells him he must born again. Why would Jesus tell him he must be born again? What was the first birth?
And thou shalt say unto Pharaoh, Thus saith the LORD,
Israel is my son, even my firstborn: (Exod. 4:22 KJV)
God says that Israel is His firstborn. So Israel is God son, born of God, metaphorically. Born as the people of God. Born as the people of promise. The promise to Abraham. God told him that He would make him the father of a great nation. That He would give the land to Abraham and his seed. Yet, the Israelites are not the recipients of the promises. How can that be? They must be born again. But, God promised the land to Abraham and his seed. Israel is Abraham's seed. The apostle Paul clears up this confusion for us.
16
Now to Abraham and his seed were the promises made.
He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. (Gal. 3:16 KJV)
So, the Israelites understood seed to be plural, they as the offspring of Abraham would receive the promises to Abraham and his seed. However, Paul tells us that they misunderstood the text. When God said seed, He didn't mean seed plural but rather He meant seed singular and that seed is Christ. Christ is the seed that would receive the promises, not the all of the Israelites. Nicodemus, your physical birth as the seed of Abraham is not sufficient to gain you access to the kingdom. There is a different way. You must be born of water and that spirit. Water is the entrance to the New Covenant, baptism. What does it mean to be born. Is it not to come into being, to come alive? What is resurrection? Is it not to come into being, to come alive? If the body returns to the dust then has to come together and come to life a second time. Is it not being, "born again"?
The Israelites would have to be born of God a second time in order to see the kingdom. They were born of God once, "Israel is my son". However, they would need another birth from God. They would have to become believers in Christ. What does John said about the believer?
Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.1 (1 Jn. 5:1 KJV)
John says that whosoever believes is born of God. The Gentile who believes is born of God, the Israelite who believes is "born again" of God.