"Nicodemus answered and said unto Him, How can these things be?" And, in fact, in the carnal sense, he knew not how. In him occurred what the Lord had said; the Spirit's voice he heard, but knew not whence it came, and whither it was going.
Long discourse nothing profits him who understandeth not a whit. Wise then is the saying in the book of Proverbs, Well is he that speaketh in the ears of them that will hear. And this the Saviour showed by trial to be true, giving Himself an ensample to us in this too. For the teacher will be wholly free from the charge of not being able to persuade, saying what himself thinks good, though he profit nothing by reason of the dulness of the hearers. Besides we learn by this, that hardness in part is happened to Israel. For hearing they hear and understand not.
Nicodemus cannot take in the mysteries of the Divine Majesty, which our Lord reveals, and therefore asks how it is, not denying the fact, not meaning any censure, but wishing to be informed: Nicodemus answered and said unto Him, How can these things be?
[AD 430] Augustine of Hippo on John 3:9