John Chrysostom — as excerpted in the Catena Aurea on John 3:4-8

“(Hom. xxiv. 3) Nicodemus coming to Jesus, as to a man, is startled on learning greater things than man could utter, things too lofty for him. His mind is darkened, and he does not stand firm, but reels like one on the point of falling away from the faith. Therefore he objects to the doctrine as being impossible, in order to call forth a fuller explanation. Two things there are which astonish him, such a birth, and such a kingdom; neither yet heard of among the Jews. First he urges the former difficulty, as being the greatest marvel. Nicodemus saith unto him, How can a man be born when he is old? can he enter a second time into his mother’s womb, and be born?”