“That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.”
Here we can refer back to John 1:12-13 where John had distinguished natural birth from being ‘born of God'. Being born a Jew, or in Christendom, or in a Christian family is not sufficient. Just as being baptised is not enough. New life received from the Spirit is what is required, God watering the heart. This comparison of flesh and Spirit arises, of course, from Nicodemus' earlier question. Having made clear that He is referring to the Spirit under the picture of life-giving water Jesus has now connected it up with what Nicodemus has asked.