All the efforts of Old Testament Law-keeping did not provide anybody with the adequate righteousness God was looking for. The Kingdom of God is the world of the Holy Ghost . It is a spiritual kingdom , and not a physical one. It does not concern our being physically born into a physical world. It is one's human spirit that is born again, not one's flesh.
Flesh only gives birth to flesh.
It is not flesh that experiences this new birth experience. And it is not flesh which gives birth to what Jesus is speaking about. Jesus said we must be born of the Spirit, not of the flesh. And Spirit does not give birth to flesh. It gives birth to a human spirit.