Nicodemus said to him, "How can this be?" Jesus answered him, "Are you a teacher of Israel, and yet you do not understand this? Truly, truly, I say to you, we speak of what we know, and bear witness to what we have seen; but you do not receive our testimony. If I have told you earthly things and you do not believe, how can you believe if I tell you heavenly things? No one has ascended into heaven but he who descended from heaven, the Son of man. - John 3:9-13 (RSVCE)
Nicodemus asks our Lord how these things of which He speaks are to be done. Jesus tells him that he, as a teacher in Israel, ought to understand them.
The surprise that our Lord expresses at Nicodemus' ignorance is meant to teach us that if we are in any position of authority, God expects of us a higher standard of knowledge and practice than He expects of others. There is scarcely anyone who is not invested with some authority from God over children, servants, pupils, younger members of our little circle. Some of us have more important and responsible authority. Do we appreciate the account we shall have to give of the use we have made of our authority?
Jesus had explained to Nicodemus, by a metaphor from sensible things, the meaning of the new birth which the Spirit of God works in the soul. He had spoken with the Divine authority of one who had Himself seen and known that which He announced respecting the things of God. But Nicodemus had not yet the grace to understand, and so he understood not. In Divine things we can do nothing without grace. We may be able, learned, quick-sighted, intelligent, but without grace we are blind and deaf.
Our Lord furthers tells him that none can speak from direct personal knowledge of heavenly things save He Himself, the Son of Man, and though He had come down from Heaven, He was still present there, in full possession of the Beatific Vision. Happy those to whom Jesus teaches heavenly truths!
- taken from The Ministry of Jesus Christ: Short Meditations on the Public Life of Our Lord, by Father Richard Frederick Clarke, SJ