John 3:9

"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?

Nicodemus still remains with the Jewish weakness, and therefore again asks: "How can this be?"

Here the third point is touched upon, namely the manner of understanding this spiritual regeneration, which because Nicodemus did not possess, he therefore doubts and inquires; whence he says: Nicodemus answered and said to Him: "How can these things be?" And in this the diligence of Nicodemus is noted, who did not cease from inquiring until he understood. Chrysostom: "If the Jews had heard, they would immediately have departed in derision"; and therefore he who was moved through the captivation of the intellect and through faith deserves to be more fully instructed by the Lord.

Then the cause and reason for spiritual regeneration are set forth. First, a question is asked by Nicodemus; secondly, the Lord's answer is given (v 10).

It is apparent from the first that Nicodemus, as yet dull, and remaining a Jew on the level of sense, was unable to understand the mysteries of Christ in spite of the examples and explanations that were given. And so he says, How can all this happen?

There are two reasons why one may question about something. Some question because of disbelief, as did Zechariah, saying: "How will I know this? For I am an old man, and my wife is advanced in age" (Lk 1:18); "He confounds those who search into mysteries" (Is 40:23). Others, on the other hand, question because of a desire to know, as the Blessed Virgin did when she said to the angel: "How shall this be, since I do not know man?" (Lk 1:34). It is the latter who are instructed. And so, because Nicodemus asked from a desire to learn, he deserved to be instructed.