John Chrysostom — as excerpted in the Catena Aurea on John 3:1-3

“(Hom. xxiv. 2) He says therefore, Except a man be born again, he cannot see the kingdom of God: as if He said, Thou art not yet born again, i. e. of God, by a spiritual begetting; and therefore thy knowledge of Me is not spiritual, but carnal and human. But I say unto thee, that neither thou, nor any one, except he be born again of God, shall be able to see the glory which is around me, but shall be out of the kingdom: for it is the begetting by baptism, which enlightens the mind. Or the meaning is, Except thou art born from above, and hast received the certainty of my doctrines, thou wanderest out of the way, and art far from the kingdom of heaven. By which words our Lord discloses His nature, shewing that He is more than what He appears to the outward eye. The expression, From abovea, means, according to some, from heaven, according to others, from the beginning. Had the Jews heard it, they would have left Him in scorn; but Nicodemus shews the love of a disciple, by staying to ask more questions.”