Old Nicodemus was so impressed by Our Lord’s words and actions that he was willing to credit Jesus as having come from God. But Jesus was saying that something more than “willingness” was required of Nicodemus, and he likened that “something more” to being born again.
Nicodemus, taking Our Lord’s metaphor literally, asked how could an old man like him get back into his mother’s womb to be again born.
Jesus told him the that the rebirth he was prescribing for Nicodemus was not a bloody, painful birth, but one that comes about as noiselessly as the passing of a breeze.
Newly discovered notes from the First Century let us see that what we now see as the Sacrament of Confirmation was an integral part of Christian initiation. Just as the baptized person was coming up from the water, the bishop was there to anoint him with the Spirit. Tat was “being born again of water and the Spirit.”
I like quoting from George Herbert’s poem “Decay” where he pictured as weak Christians as walling off the Spirit into just part of his heart.
“Now thou dost thyself immure in one small corner of a feeble heart; where yet both Sin and Satan, thy old foes, do pinch and straighten thee, and use much art to gain thy third and little part.”