John 3:6

That which is born of the flesh is flesh - This is the answer to the objection made by Nicodemus in John 3:4 . Can a man enter the second time into his mother's womb and be born? Our Lord here intimates that, were even this possible, it would not answer the end; for the plant will ever be of the nature of the seed that produces it - like will beget its like. The kingdom of God is spiritual and holy; and that which is born of the Spirit resembles the Spirit; for as he is who begat, so is he who is begotten of him. Therefore, the spiritual regeneration is essentially necessary, to prepare the soul for a holy and spiritual kingdom.